It doesn't subtract the power if your power damage is bigger than the remaining power of the ship.
Example
- 50 000 power in ship, gets hit by a 10 000 power reduction resulting in 40 000 power -> works
- 5 000 power in ship, gets hit by a 10 000 power reduction resulting in no power reduction, the regen does stop for a second. -> doesn't work
If I for example had a EMP weapon that hits 1 million each projectile I could not drain any ship that has less than 1 million power storage.
How to reproduce:
# make an emp weapon with roughly a total of 500 blocks
# add some power cap to a shipcore
# give it power with /power drain -200000
# shoot at it and notice that at some point you stop draining since your damage > remaining power