zozatos

Basically, what he was talking about is that KSP does not model ullage. When a rocket motor cuts out, the rocket is now under zero acceleration. This means that all of the liquid fuel in the tanks just floats around. Getting gas into the turbopumps that fuel the rocket is bad for them, so generally there will be small engines called "ullage motors" which push the rocket up, which pushes the fuel inside the tank downwards (towards the main engine plumbing). Then the main engine can start. With "Fire in the hole" either the explosive bolts from the stage separation act as your ullage motors, or the previous motor is still running and provides ullage. Then you start the next stage engine without having to carry ullage motors. Thus efficiency.