I think that one of the worse ways to train "short power" is to do it in combos and the reason is that it tends to deteriorate into "slappy hands" with very little penetrative power.
A good way is to use the lead hand since people tend to catch on quicker with that one and extend it forward till it is touching a target ( a pad or mitt is best at this stage) with its finger tips and then relax and drive through the target as you close your fist. Allow the impact to "recoil" your hand back.
You should be about 75% extended at this learning stage so that means that you are really driving just the last few inched of the extension of your arm and the real penetration comes from the whole body movement going forward into the target.
All this is not the easy to explain in words but if you see it you can catch on pretty quick with practice.
Doing it off the rear hand is a tad harder to pick up but sometimes doing it as a 1-2 combo helps a bit.
Once you have it down on the pads move to the HB.
Of course translating this into sparring is a tad more difficult since your partner is not just standing there and he is/should be hitting you back but the crucial element is getting the whole body into because once you have that down then ANY strike whether short distance or not, will always have the nice "english" on it at the end.
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !