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

You can force a crit:
https://puu.sh/EInGt.png

Can't force a crit: You need to shoot your weapon to build damage up.
https://puu.sh/EInGP.png
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.
https://puu.sh/EInJa.png

Crit banned. You were shooting too many critical shots.
https://puu.sh/EInMk.png

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