Q:

Understanding Decay function and its parameters in ElasticSearch

I was reading about decay functions in ElasticSarch  in order to promote more recent results 
If I define decay function as following:
"DECAY_FUNCTION": { 
"FIELD_NAME": { 
"origin": "2013-09-17", 
"scale": "10d", 
"offset": "5d", 
"decay" : 0.5 
 } 
}

How the score will be affected within the Offset , Scale regions and outside them?

A:

Depending on this reference query-dsl-function-score-query I will answer my question:
Let's take this slope as example:

Here the origin is age=40 and we have offset=+-5 years. So the decay function will return value of 1 inside offset [35-45].
Inside Scale the function will return the value of decay(which is in our example 0.5)
The regions outside Scale(which are more than 50 and less than 35), decay function will decrease further towards 0 value.
PS

The scale parameter just steers how quickly the function approaches 0
The decay function never returns a negative value. It will always be  between 0 and 1.
There are three types of decay function (gauss, exponential, and linear) and as we noticed in the slope above how each type steers.

