what is never to be engaged when using power tools?\no trigger lock\no bit shank\no booster

Brief Explanations:

The trigger lock is a safety - feature to prevent accidental start of power tools and should be engaged when not in use. The bit shank is a part of the tool that holds the bit and is engaged during normal use. A booster is not a standard feature that should never be engaged. So, the bit shank and booster are parts or features that can be engaged during normal operation, while the trigger lock should be engaged when not using the tool, but never engaged during actual operation to allow the tool to function.

Answer:

trigger lock