We do it as part of any regular class after we practiced the last technique (genereally suwariwaza kokyuho) and we do it whith our current training partner (be him/her small/tall/heavy/thin...)
What we begginers are always told is the point that you should put (as Eric pointed out) your butt below his/her's kind of to lower your center.
And you should start straightening your knees once your partner is almost laying on your back, not before.
So far I never heard of anyone beeing injured by this practice (I hope I never hear)
I find it a very relaxing way of ending class.
It helps you to strech your muscles so you don't end up all rigid once you stop practicing.