Neo/Ben has a good start on describing it. You are in a position that resembles how a lady would squat to pee in the woods. You then have to move from one foot to the other and "walk" to the other side of the room and back again (at least that is how it was when I had to do it). The "walk" requires you to almost completely extend your leg - you end up walking on your heel - it is painful on hard tile floors. ElPensador, it is probably worse than what you imagine.
When I went through we had this one dude, that for the life of him, could not do it. Everyone else was finished and the doc kept having this guy try again - he always fell on his butt - if it wasn't so sad, it would have been funny.
To answer Warlord's question, MEPS is usually located in the Feral Building in whatever large city you reside in or around. I don't know if this is universal, though. Flight physicals are usually at an airbase. You will have to have an interview with a flight doc as part of your physical.