Turn-in point
The turn-in point is where you begin steering into a corner, which largely decides where your apex and exit will be.
Also called: turn-in, turn-in marker
The turn-in point is the moment you start to turn the steering wheel for a corner. It comes after or overlapping the end of braking, and it largely decides everything after it: turn in early and you apex early; turn in late and you apex late.
A good turn-in is one smooth, deliberate motion at the right rate, not a series of corrections. You place the car on the outside edge of the track, pick a fixed marker, and begin steering when you reach it. The car should take a set and arc toward the apex.
The most common mistake is turning in too early because the corner looks close. Another is turning in too sharply, overloading the front tires. Pick a reference point and adjust it a little at a time, judging the effect on your exit.
A consistent turn-in point is what makes the rest of the corner repeatable and predictable.
Simplified 90° corner. Real corners trade entry, mid-corner and exit speed differently; what follows the corner decides which matters most.