I wondered that too but remembered that I could only roll it up so far and it seemed to lock in the down position too!. so I looked at the gear (da biggie)
it has a stop built in for down, actually it just doesn't have any more teeth ground in so the gear can't move down any more. and there is an adjustment stop on the regulator itself(little round slug) that stops it on the top stroke. I guess the top adjustment is for people who (like me) crank it up hard to seal it up and bend things.
