The text in the crit bucket bar tells you how good your crit ratio is.

You can force a crit:

Can't force a crit: You need to shoot your weapon to build damage up.
The red bar shows you how much damage you have to do in order to crit again.

You can't randomly crit with this weapon.

Crit banned. You were shooting too many critical shots.

The best way to use the crit hack is to have a key bound and use it whenever needed.