Q:

Why do some people only work well under pressure?

It is  common for people to procrastinate. Putting off tasks until  the resulting deadlines get closer and  there comes a point at which the person will fire into action.
(whether this point leaves the person with enough time to complete task is another question).
Why is it that some people seem to only work well under pressure? Why is it that people find it difficult to discipline themselves and have all their task complete well before deadlines are due?

A:

I have often wondered about this, as I know I do this, even setting myself arbitrary deadlines, not always with the best results.  The following is not an opinion, but based on some information found while, ironically, taking a 'break' from my PhD drafting.
According to "In Search of the Arousal Procrastinator", (Pychyl, 2008), the notion of 'working under pressure' could be based on their cognitive dissonance, concluding (in part):

a common theme about self-deception and task delay. We often rationalize our current delay because we don't feel like  working on the task by evoking (irrational) beliefs like, "I'll feel more like it tomorrow" or "I work better under pressure." Although this may be the case on rare occasions for some tasks, on the whole these thoughts are simply rationalizations to justify further delay and make us feel good in the short run.

However, according to "Procrastination or 'intentional delay'?" (Novotney), there could be benefits in the procrastination, as it may lead to better planning of what is needed to be done, so the pressure of the deadline is not necessarily entirely harmful.
Other resources cited in the above links:
Ferrari, J. R. (1992). Psychometric validation of two procrastination inventories for adults: Arousal and avoidance measures. Journal of Psychopathology and Behavioral Assessment, 14, 97-110
Ferrari, J.R., Johnson, J.L., McCown, W.G. (1995). Procrastination and Task Avoidance: Theory, Research, and Treatment. New York: Springer.
Schouwenburg, H.C., Lay, C., Pychyl, T.A., Ferrari, J.R. (Eds.) (2004). Counseling the Procrastinator in Academic Settings. Washington, DC: APA.  
Burka, J.B., & Yuen, L.M. (2nd Edition). (2008). Procrastination: Why You Do It, What to Do About It Now. New York City: Da Capo Lifelong Books.

A:

I think it is often a myth that people work better under pressure. Most people who say they work better under pressure have not really tried working under normal conditions. Even if a person thinks he works better under pressure, he may work better under normal conditions. That being said, pressure (more specifically time pressure) helps in providing closure to tasks.
One of the main reasons for procrastination has to do with mental construal theory. Basically goals that are far from a person in time, space or both seem very abstract. So it is difficult to initiate abstract goals. Goals that are close in time and space are more specific. Thus they are easier to implement. To make abstract goals more concrete one should use implementation intentions and mental contrasting. That is, form specific "if then" plans to initiate a goal. Mental constrasting provides both a idealistic and realistic picture of your goal accomplishment.
As an example, suppose you want to get started on some assignment. Mental contrasting would mean that you visualize yourself completing the assignment and the feelings associated with it (e.g. happiness, pride, etc.). Then you would visualize the obstacles preventing you from reaching the goal (e.g. feeling tired, hungry, good tv show on). This provides a sort of "energization" to initiate action. Then you would form the implementation intentions based off these obstacles. However, forming too many implementation intentions could be counterproductive. Note that this assumes that the goal intention is strong in the first place.
Sources
Simpson, W.K., & Pychyl, T.A. (2009). In search of the arousal procrastinator: An investigation of the relation between procrastination, arousal-based personality traits and beliefs about procrastination motivations Personality and Individual Differences, 47, 906-911.
The effect of temporal distance on level of mental construal
Nira Liberman,a,* Michael D. Sagristano,b and Yaacov Tropeb
W Leroy, S. (2009). Why is it so hard to do my work? The challenge of attention residue when switching between work tasks. Organizational Behavior and Human Decision Processes. Vol. 109. pp 168-181.

