user17744: Enlightened people doesn't have any desire, because they see impermanence in everything. All the things are useless to them. To achieve enlightenment , one required to practise deattachment, uncertainty through out the samsara. So , Is understanding a practise ?

If it is: We loose skills when we don't do somethings for long time. But enlightenment can not be undone.
If it isn't: When we do vipassana meditation for a long time, we started to see impermanance in every day life. We loose attachments to lot of things . What is happening there ?

I'm not sure I express and structured the question as I want . So Improves are well come.

user295: 
OP: So , Is understanding a practise ?

Understanding arises through the practice.

OP: If it is: We loose skills when we don't do somethings for long time. But enlightenment can not be undone.

Yes. As enlightenment is understanding of reality as it is and this does not change in light of new experiances.

OP: If it isn't: When we do vipassana meditation for a long time, we started to see impermanance in every day life. We loose attachments to lot of things . What is happening there ?

Attachment leads to pain. When you understand this through Vipassana you do not get attached.

user8157: There is wisdom that is practised to develop & support samadhi (concentration) and wisdom that arises from the practise of samadhi (concentration), as described below: 

Of those, right view is the forerunner. And how is right view the forerunner? In one of right view, right resolve comes into being. In
  one of right resolve, right speech comes into being. In one of right
  speech, right action... In one of right action, right livelihood... In
  one of right livelihood, right effort... In one of right effort, right
  mindfulness... In one of right mindfulness, right concentration... In
  one of right concentration, right knowledge... In one of right
  knowledge, right release comes into being.
Maha-cattarisaka Sutta

The wisdom or wise reflection that is practised to develop & support samadhi is called Right Understanding/Right View and sampajjana. This pre-learned or pre-experienced wisdom is brought to and kept in mind by the practise of mindfulness (sati). 
The wisdom or direct seeing that arises from the practise of samadhi is called vipassana (insight), samma nana (Right Knowledge), etc. 
Note: vipassana is not a 'technique, method or practise'; as commonly taught. As already said, vipassana is a result of practice, as described below: 

Having thus developed (practised) the noble eightfold path...these two qualities occur in tandem: tranquillity (samatha) & insight (vipassana).
Maha-salayatanika Sutta


user-1: If you've never seen the color red before and someone finally points it out to you, you'll never again be able to unsee the color red.  You don't have to practice to keep seeing red.  Instead, you begin to see it everywhere. You even begin to see different shades - raspberry, scarlet, vermilion - and how they differ from each other yet still can be classified as the color red.  
Seeing impermanence works exactly the same way.  An untrained person, even one who's read every book on Buddhism on the planet, has ultimately no idea what impermanence actually is.  One day, after many hours of meditation, maybe that person directly sees impermanence for the first time.  From that day forward, he can't unsee it.  It begins cropping up everywhere.  He doesn't have to practice seeing impermanence, it just becomes an undeniable feature of his existence.  After many more hours on the cushion, perhaps he even begins to see impermanence with even more subtlety - how it ties into other facets of the path, how it powers his meditation, how it liberates him from his fear of death.  Once these these things are perceived, they don't go away.  Even if they are forcefully ignored, they are like irritants...sand in your underwear.  (That's why once you start, it's best to finish!)             

17744: <p>Enlightened people doesn't have any desire, because they see impermanence in everything. All the things are useless to them. To achieve enlightenment , one required to practise deattachment, uncertainty through out the samsara. So , Is understanding a practise ?</p>

<ol>
<li>If it is: We loose skills when we don't do somethings for long time. But enlightenment can not be undone.</li>
<li>If it isn't: When we do vipassana meditation for a long time, we started to see impermanance in every day life. We loose attachments to lot of things . What is happening there ?</li>
</ol>

<p>I'm not sure I express and structured the question as I want . So Improves are well come.</p>
 295: <blockquote>
  <p><strong>OP: So , Is understanding a practise ?</strong></p>
</blockquote>

<p>Understanding arises through the practice.</p>

<blockquote>
  <p><strong>OP: If it is: We loose skills when we don't do somethings for long time. But enlightenment can not be undone.</strong></p>
</blockquote>

<p>Yes. As enlightenment is understanding of reality as it is and this does not change in light of new experiances.</p>

<blockquote>
  <p><strong>OP: If it isn't: When we do vipassana meditation for a long time, we started to see impermanance in every day life. We loose attachments to lot of things . What is happening there ?</strong></p>
</blockquote>

<p>Attachment leads to pain. When you understand this through Vipassana you do not get attached.</p>
 8157: <p>There is wisdom that is practised to develop &amp; support samadhi (concentration) and wisdom that arises from the practise of samadhi (concentration), as described below: </p>

<blockquote>
  <p><em>Of those, <strong>right view</strong> is the forerunner. And how is right view the forerunner? In one of right view, right resolve comes into being. In
  one of right resolve, right speech comes into being. In one of right
  speech, right action... In one of right action, right livelihood... In
  one of right livelihood, right effort... In one of right effort, right
  mindfulness... In one of right mindfulness, right concentration... In
  one of right concentration, <strong>right knowledge</strong>... In one of right
  knowledge, right release comes into being.</em></p>
  
  <p><em>Maha-cattarisaka Sutta</em></p>
</blockquote>

<p>The wisdom or wise reflection that is practised to develop &amp; support samadhi is called Right Understanding/Right View and sampajjana. This pre-learned or pre-experienced wisdom is brought to and kept in mind by the practise of mindfulness (sati). </p>

<p>The wisdom or direct seeing that arises from the practise of samadhi is called vipassana (insight), samma nana (Right Knowledge), etc. </p>

<p>Note: vipassana is not a 'technique, method or practise'; as commonly taught. As already said, vipassana is a result of practice, as described below: </p>

<blockquote>
  <p><em>Having thus developed (practised) the noble eightfold path...these two qualities occur in tandem: tranquillity (samatha) &amp; insight (vipassana).</em></p>
  
  <p><em>Maha-salayatanika Sutta</em></p>
</blockquote>
 -1: <p>If you've never seen the color red before and someone finally points it out to you, you'll never again be able to unsee the color red.  You don't have to practice to keep seeing red.  Instead, you begin to see it everywhere. You even begin to see different shades - raspberry, scarlet, vermilion - and how they differ from each other yet still can be classified as the color red.  </p>

<p>Seeing impermanence works exactly the same way.  An untrained person, even one who's read every book on Buddhism on the planet, has ultimately no idea what impermanence actually is.  One day, after many hours of meditation, maybe that person directly sees impermanence for the first time.  From that day forward, he can't unsee it.  It begins cropping up everywhere.  He doesn't have to practice seeing impermanence, it just becomes an undeniable feature of his existence.  After many more hours on the cushion, perhaps he even begins to see impermanence with even more subtlety - how it ties into other facets of the path, how it powers his meditation, how it liberates him from his fear of death.  Once these these things are perceived, they don't go away.  Even if they are forcefully ignored, they are like irritants...sand in your underwear.  (That's why once you start, it's best to finish!)             </p>
