Excellent book packed with info, tricks, highly illustrated--will save you time/money.
Locking carabiners for indoor and sport rock climbing. The dual pin and auto locking carabiners use a spring mechanism to snap in a clocked position, while the screw gate carabiner locks using threads which must be twisted into a locked position. A lock-type carabiner is particularly useful for climbing applications.
The locking mechanism creates a secure anchor which is much more difficult to accidentally open. This can happen during belaying and anchoring activities if the climbing rope is twisted or the carabiner is pressed against a rock or other gear. The locking mechanism provides added security. Typically, climbers need 2 or 3 locking carabiners and a belay device for anchoring gear and belays.
The locking carabiner has sleeve that slides or screws over the gate which locks it closed. It has an "auto locking" spring loaded screw sleeve, or a threaded screw gate. The spring sleeve rests in the closed position. The climber must twist the sleeve to open the gate. Twist lock sleeves are easy to use and can be operated with one hand. These are popular for indoor and sport climbing.
Screw gate type locks can also be operated with one hand. When locked, the sleeve screws over the latch preventing the gate from opening unless it is unscrewed. This type is popular with outdoor, canyon and big wall climbing. They considered by most climbers to be the most secure type of locking carabiner. These are slightly larger than the non-locking version which makes it applicable to use for connecting two ropes together, or connecting a rope to a runner. The locking type can be found in all of the same shapes as the snap type: Oval, D-symmetrical and A-symmetrical.
Belaying, rappelling and abseiling are the most common uses for locking carabiners. The largest of the locking types are the most popular as it provides more room for the rope and sling, or two ropes. Plus the additional space gives working room for fingers.