Calcium channel blockers (CCBs) in wide QRS vs narrow QRS

I have learned that CCBs is the first line agent in controlling of narrow QRS cuz it blocks the number of impulses that arrive at ventricular tissue, thus reducing the heart rate However, it is not recommended for wide QRS cuz it may increase the impulse arriving at ventricular tissue, so speeding the heart rate.
I am a lito confused! Why increasing and reducing?
Any thoughts? Much appreciated!