It is also important to stress the fact that any blocking of the device or the cam will negate the blocking action on the rope. This is why it is essential to avoid holding the device with your whole hand, to avoid keeping your thumb constantly on the cam, to avoid blocking the cam...

It is difficult to make an exhaustive list of all incorrect techniques. Here are some illustrations:

Note that the use of incorrect belay technique is the primary risk factor in an accident, especially when the belayer is surprised by a fall. If you recognize yourself in any of these illustrations of incorrect technique, adopt the technique presented at the beginning of this paragraph.

【轉(zhuǎn)自：https://www.petzl.com/US/en/Sport/Belaying-with-the-GRIGRI?ActivityName=Rock-climbing】