Hmmm... This sounds like something my spin instructor does. She calls them "singles." Basically, we're spinning along, out of the saddle, and then moving our weight forward for a couple of pedal strokes, then back for a couple, then forward... repeat. I never really feel like I'm getting any benefit from this, but then I'm not really sure what it's supposed to do. If this is the same thing you're doing in your class, my instructor says it's supposed to work your abs, add a little bit of arm stability back into it (usually we aren't putting any weight at all in the arms), go thru different muscles in the legs (instead of focusing on the same ones), and basically just make the sprint a little less tedious. Personally, I think she does it for the last reason, to keep us all excited, focused on her, and trying hard & following along.
My instructor is a former pro mtn biker... I trust whatever moves she does as good ones. *shrug*
Fast = Fun