In this short video, a rock gym employee discusses how to use an auto-belay device.
There are a couple of important things that he mentioned in the video:
First, remember to actually clip into the auto-belay carabiner. There's no one there to check you, so you have to check your harness and carabiner yourself. If you don't clip in properly -- or at all -- you may get hurt.
Most auto-belay accidents happen because the climber forgot to clip into the belay.
Second, if you accidentally let go of the carabiner, don't worry about it. Don't try to climb up and get it. Just tell a staff member. These things happen all the time.
Third, the first few times you use the auto-belay it will be very scary. It doesn't rally catch you until you've fully weighted it, and so it can feel like you're falling for a moment before it engages. There's value in getting used to this close to the ground.
Auto-belays are great. I personally really appreciate it when gyms have this option available...
--Jason D. Martin