In an ideal world, you’d never have to slow down in a corner. You’d judge it perfectly, turn in with one push, then swoop elegantly through on a constant positive throttle. But the world ain’t perfect. There will be times you need to lose speed in a corner. Sometimes, in a hurry.
It often happens when chasing the vanishing point. You see the corner is tightening, so you need to slow down. Rolling off the throttle is the first thing to do. As well as engine braking slowing the bike, it’ll also tend to tighten your line.
Applying a little rear brake in a corner will have a similar effect, slightly more pronounced. But you need to be gentle to avoid lock-up or a skid.
If it gets so you have to use the front brake, your difficulties increase. It’ll put weight on the front, which is not good for stability. The bike will have a tendency to sit up and run wide, so you’ll need to resist this with countersteering and your bodyweight to hold your line. Finally, you run the risk of overwhelming the front tyre’s grip, resulting in a lowside crash.
If you have room, it may be better to sit the bike up and brake as hard as you can in a straight line, before steering back into the bend.