Quite often playing behind the beat is confounded with getting slower...as for playing in front of the beat it`s just often vice versa.
Though it has to do with feel, not neccessarily with the tempo, at least not consciously.
Playing in front of the beat means to push it but without actually getting faster. You just extra-push it and these are just miliseconds which decide whether you play in front of the beat, on top of it or behind it.
This depends on the musical situation and on the musician.
Imagine that you play a straight swing for the big band. You might wanna push it and play in front of the beat to drive the whole thing. In my opinion the smaller the Jazz setting, the more you get the opportunity to relax and lay back a bit...if there`s no intention to extra-push it. The swing gives a good example, too:
Some drummers play it wide and open, others play it with little air...
If you`d like to I can recommend to listen to Billy Higgins here. Especially on the tune "Do I Crazy?" with John Scofield you hear this wide behind the beat-swing which almost creates a Latin feel sometimes.
http://www.drummerworld.com/Sound/billyhigginsdoicrazy.mp3
Then listen to this version of CTA by the Brad Mehldau trio with Jeff Ballard on drums. Completely different world.
http://www.youtube.com/watch?v=iLQYwoPaTb4&feature=related
In the end playing in front, behind the beat or in between that depends on the whole band as well and surely mood and athmosphere aren`t unimportant.
Hope that helps a bit,
Karl