[Christ]

slip shifting = when you are letting off the gas, apply pressure to the stick as though you're shifting gears. You'll feel it slip out of gear smoothly, and it won't damage anything.

As you're coasting, you kinda get a feel for when you have to put it back in gear and start accelerating again.. so you start to slowly rev the engine to the speed you'd need to smoothly put it back in gear, no grinds, no clutch use.

The second part is called speed-matching.

And slip-shifting in it's real terminology is exactly what everyone else calls "half-clutching" it's still using the clutch, but not fully disengaging it, forcing it to slip while shifting to another gear, but not losing as much inertia. It's a ricer thing, although, I'm sure it had real-world application somewhere.