What do you think the best way to get power in your uppercuts to body/bodyshots it?

I was talking to one of my coaches about it last night.

He basically said he won't tell me how to do it I've just got to get on the bag and find my rhythm there.

He did tell me while doing pad work that I'm trying to bend at the knees too much - I should bend at the hips instead and throw my weight into it so if I miss I will fall over -

Only I feel weird doing that coz - what if my opponent moves?!?

I know there is something to what he's saying though - finding your own rhythm etc to get power.

So my Q to all the trainers is - how do you get power in the uppercut and to what point do you agree with my coach - that i just have to flog the bag more?

Thanks*