[bcj]

In auto the controller will usually coast when you let off the gas until you call for acceleration again, then figure out what gear is most appropriate.
You'll need to hit the brakes if you want to slow down.

In "manual" you can use engine braking to scrub off speed without putting more, or any heat into the disks, saving them for even more deceleration later.

When you have a known section of road or track, you can decide which single gear you want for that bit of it.
Use the engine to modulate accel or decel to match the corner conditions.

The auto would want to flip up a gear or two when you let off the gas.