Huh, the ones I've used (at the Westway in London) don't have a "stop" at all -- if you weight them, they slowly lower you.

Which is part of what seems to give everyone a moment of the Fear at the top, because you can't feel a belayer take you; you just have to let go and trust the machine not to drop you.

It's unnerving initially, but I found I adjusted rapidly. They're excellent for training endurance (at least if no-one else is queuing to use them, and if they are then that's someone else without a partner who might be willing to pair up ...).

Not being able to rest/lean on the rope is frustrating if you want to repeatedly work moves on something, but OTOH it does keep you honest and means you can't cheat too much. *g*