The main breaking mechanism on the stop...
Braking, braking. Let's hope there is no "breaking" mechanism on any descender.
I would recommend starting with something other than a Stop and once you have rappelled some borrow one and decide if you want to switch.
I second this recommendation. And...I recommend learning how to rappel on a simple friction device to develop the correct braking reflex and technique*. Yes, this means NOT learning on a brake bar rack.
Simple devices include figure 8s, ATCs, standard bobbins, etc. (even a munter hitch). A top belay is *required* during training for all beginners. It may be somewhat difficult to find a caver who is competent in belay techniques, but such a caver will likely possess above-average skill and safety consciousness. You will probably want to find a supple rope to train on b/c many simple rap devices don't work well with super stiff caving rope. Too many cavers become entirely dependent on the rack from the start and have difficulty using/learning other devices/techniques that can be advantageous in certain situations.
Once the braking reflex is developed, THEN try out more sophisiticated devices such as the rack or STOP. Note that one reason the rack is so highly favored in TAG is the common use of PMI Max Wear rope. This rope works very well with the rack due to its stiffness. A more supple rope offers less friction with the rack, but works better with the STOP (YMMV).
*descent control via tensioning the rope below the device with the brake hand.