Polaris 180 - the little wheel on the side does what?

I took my Polaris 180 in to the pool store to get some parts, and the guy there said the little wheel on the side wasnt engaging the bigger wheels correctly. I can see that the little wheel can move a bit in two directions, per the blue arrows on the picture below. There is also a spring-loaded lever that has a notch in it that seems to have locked it into place, barely visible at the end of the red arrow.

What does this wheel actually do, and does it do it only while its running? Should the lever with a notch not have that notch - is the notch there just because of wear?

Thanks,

TW
http://i18.photobucket.com/albums/b1...polaris2-1.jpg

PS: And how can I set Firefox to not switch to search mode whenever I type an apostrophe when composing a thread?

EDIT: Apparently the lever is not supposed to have a notch in it. There's a repair kit for that part that is an easy fix. But I still can't tell what it does. The pool store guy said the little wheel is the drive wheel, which is completely wrong.

I'm surprised this unit has worked at all since we bought the house in April. I just replaced the back-up valve, which has never worked due, it turns out, to broken gears. I wondered what that thing was, I thought it was just some 'special' float thingy. The tail had worn down several of the rings to the point that the hose had two extra holes in it, and the end of tail assembly thingy was completely gone. And then the stationary float took on water, after I fixed the support bracket. When it rains it pours...

But it's back to cleaning like a champ now.

EDIT 2: Is it possible that the containment bracket (pointed to with purple arrow) is located too low? I can see that if the swing arm for the little wheel could go a little higher, then the little wheel would engage the big front drive wheel and transfer drive to the rear big wheel.