Med jumping is the following:
(Warning: don’t do this with people u don’t know)
- See a teammate in need of a revive.
- Run up to him and drop a gren.
- Revive him.
- Jump back and watch him sail up into the air about 30 feet.
Some people try to use this as an advantage during gameplay to overcome obsticles. (i.e. jump someone over the wall on mp_beach) While this is kind of cheating, the person landing takes damage (sometimes gibbing
) and the two peeps usually get ambushed before they can pull it off. So you usually see it during warmups, and when you’re just goofing around on a map.
As far as the jumping goes it’s basically what the mapper forgot to prevent. I dunno if they planned for it or not, but some of the basic maps that came with RTCW have the best jumping spots. mp_depot and mp_base have some very cool spots to get into. 
Hope that clears things up.