well you probably do have a little drag but not enough to drive you forwards but enough to stop the plates moving left or right.
look
10 men are lined against the wall and the wall is vertically corrugated. the guys have to leave the wall to the left or right by getting over each ripple best they can. whats stopping them? 7 fireman with firehydrants going flat-out.
now you get in the deep of a ripple, how are you going to get out with all that force at your back?
ya dig dude?
That is by far the best explanation i have heard, and it made me smile 8)