Ghosting would be a fake image that is 100% opaque of where the player used to be, lagging behind by 1 second, whereas the actual player's position would be a 90% cloaked model of the player at that location. The 90% cloaked model would be hard to hit and aiming would be disorienting for marines because of the fake model.
This would primarily help lerks in vent, as gassing a marine busy area is nearly impossible if everyone is watching the vent. So the 90% cloaked player model would pop out, gas, then pop back in without extreme danger of being seen every time. Of course it would help skulks as well.