The PDCs have counter-thrusters when they're firing
Just noticing this for the first time. When the PDCs fire, there is a counter-thruster to keep the recoil from affecting the ship's attitude / maneuverability. I love little attentions to detail like that.
Thanks I'll look out for this when I re-watch this show again. So much thought behind all the details
Whenever the Roci's railgun is fired, it's accompanied by a momentary burn of her main drive to cancel out that impulse.
Except that time they use the rail gun as propulsion..
That was around Ilus when they're trying to tow the Barb to a safer orbit because the protomolocule changed the local laws of physics such that all of the ships' fusion drives no longer worked. The drive could have tried to compensate for that, but with fusion not working, it wouldn't have.
Plus, they could probably disable that somehow if they needed to (or Naomi or Amos could jerry-rig it for the same effect).
I just caught that, though not in action (will keep an eye out for it though). Just started season 4, and Alex mentions when it's getting installed that it needs to be synced with the engine or else they're gonna get thrown to the ceiling every time it fires (or something along those lines).
I wonder if it’s just the excess gas from the fired round
Very possible it is. With all the other attention to detail shown throughout the series, I would have to assume that's both intentional and being used to (partially) counter the recoil. Whether it's just the excess gas being used to reduce the amount of thrust required from the maneuvering thrusters or augmented with additional thrust to fully counter the recoil, I'm unsure of.