The thing is, it's actually kind of tricky to work a left hook on a big wide heavy bag the same way you'd work it on mitts. You have to step to the side more and/or throw it palm in. I hope you've been taught how to throw it palm down to start with though. It's easier to throw a left hook on an angle/uppercut bag. On a heavy bag, I actually work it very slightly palm OUT even. Everyone had their thing.