Fast bowling is complex. It’s impossible to break it down to one part; but a crucially important part for power - and therefore speed - is your hips.
Your hips are the powerhouse of the bowling action. You have leapt to the crease, landed with a braced front leg and there is a surge of energy heading up your body (like a pole vaulter’s pole sends them over the bar).
But wait. Before the energy can get into the ball to sear it down the other end it needs to get through your body.