You need to take it step by step. If you need a lot of power to dunk it, you will stiffen up. So you need to take it gradually and see where (at what point of "tension" or speed) you start stiffening up. Usually people start muscling the movement and "pushing" like in a standing vertical jump instead of being reactive, because they're not reactive enough when trying to jump maximally (in translation - they start trying to use muscle power instead of a fluid movement to achieve the desired height needed for a dunk).
Back in the day when I was light and weak I could usually dunk just from a long run-up only. 10 steps or so, at very high speeds. Meaning, almost no actual voluntary muscle power at all, just using the speed that I have accumulated during the run-up and just converting it to vertical "speed".
But as I got stronger my run-up did transition into a acceleration-type approach, where yes, I have to decelerate and "make my steps". More muscle, less reactivity.