The short answer is: they work by the fiction part of "science fiction".
The word "pulse" in front of something makes it sound cool and means whatever the author wants it to.
You'd need to be specific.
IRL: pulse lasers require so much energy that it is not practical to supply it continuously.
So the energy is usually stored from a sane supply rate and delivered in a short burst: a pulse.
Thus, anything that uses energy in pulses would be a pulse-thing.For a plasma gun, they would usually shoot a small squirt of superheated gas by some unexplained mechanism. That's the pulse - though it would be likely that the power needed to shoot would be delivered in a pulse too.
IRL plasma is controlled by electromagnetic fields - so we would think of something like a rail gun but using gas.
All slug throwers are pulse-mode weapons - that's why they go "bang". Contenporary version use gunpowder as the energy store.
By analogy with the lasers and plasma devices, you'd still think in terms of some sort of rail-gun.
However, I've seen SF weapons using antigravity to get these effects.
Like I said - it means whatever the author wants it to.
Does that help?