This is the shoulderstand:
http://www.yogajournal.com/poses/480_1.cfm
It's not so important which you do (just that shoulderstand is easier). The idea is that you invert to allow the enema to move deeper along the descending colon, hopefully to the transverse.
Yes, it might be more effective after you've already done an enema or colema, since it will have a better chance to be held and absorbed by the walls of the intestine.