What he names "fishtailing" is what is known as "highside" in motorbike circles. When the rear wheel loses grip, it and the entire frame except the front fork moves sideways. When the rear wheel inevitably retains grip, the resulting force violently flips the whole. On a bicycle this is rare and survivable, on a 200+kg mc not very much so.

In a turn you either apply energy if that is at all possible (usually), or just steer. It is recommended to accelerate inside a turn, because that way you can better control grip on the rear due to not accidentally engine-braking (not applicable to bicycles), and because thrust on the rear wheel helps the bike into the turn (applicable to bicycles).

Braking in a turn is a last-effort thing, taken only if you are certain that it'll lessen damage from otherwise imminent crash.

Biking is traction/grip control at all times. Traction is always shared between steering and brake/accel. You do not want less control in a turn, and loss of traction means loss of steering.

So you do not ever brake in a turn.

You decelerate on a straight before it. If a turn goes bad for whatever reason, that means that a) you made a mistake entering it at the speed you did, and b) now you choose what to crash into, and then execute the crash while hoping that your protection holds, which might include hard front brake and a steering input so that you lowside feet (and bike) first into the obstacle. But not before.

So the first lesson of bike-riding is being aware at all times of available traction.

95% use of fronts follows: you only use them on straights, and there they are much more efficient than rears because the whole mass of you and bike enhances front wheel traction. cf. the wheelie.