I think I mentioned before that I allow (but don't encourage) bouncing on the ESU servers. It would be nice if some consideration is given to others though: if people complain about it just play regularly, don't run up the score by bouncing so you end up with a 7-0 game for instance, and don't bounce full time everywhere or use macros for it.
In competitive league type matches and on many other servers it is considered to be illegal so make sure to follow the rules in other servers.
The video BearBeard posted above explains it. To start you need some extra speed via grapple, impact jump, or boost then with the jump bound to the mouse wheel you can spin it just before landing and your momentum will be conserved. To continue on the next landing it works best if you hold the walk key while doing your jumps/scrollwheel.
To be clear, this bouncing / bunny hopping is an engine bug. Whether you like it or not with some teamwork some cool trick runs can be accomplished. This old video shows some: