In a hairpin the exit counts most. Use the width, brake straight, turn in late and accelerate early, without cutting off other drivers on the track.
The hairpin is the slowest corner on the track and turns the kart through about 180 degrees. The honest answer to "What matters most here?": the exit. A long straight usually follows, and the speed with which you leave the hairpin you carry down the whole straight.
The driving-literature recipe fits (car racing, so a transfer): an analysis of 60 years of motorsport textbooks describes starting with a late apex and moving the point forward step by step as long as exit speed rises [1]. Whoever turns in too early gets too tight to the inside with the kart and must re-steer at the exit [1]. In a hairpin that mistake is especially expensive.
The second peculiarity: the hairpin is a classic overtaking spot. The DMSB regulations therefore state that in the braking zone before a corner a change of direction to prevent an overtaking move is not permitted [2].
You do not win the hairpin when turning in but when driving out.
Entry criteria (practice standard):
Before a hairpin you approach on the outside so you have the full width for the arc. That is coaching practice: a wider line makes the arc less tight and you can turn in later and with less steering angle. Not every track allows that; mind the track limit.
Milestone: You approach the hairpin from the outside in five laps without touching the track limit.
You brake on the straight, not while turning in. The textbook calls braking "squeeze … progressively" [1]. You release the brake before you turn in, not only at the apex. That feels too early at first: in a hairpin the kart is very slow after braking, and you have hardly any time to build pressure. So practise the braking on the straight first until it sits.
Milestone: You brake at one marker with the same pressure curve, five laps in a row.
The turn-in point comes later than feels right. The textbook says it so: "Intentionally turn into corners later than you think you should" [1]. Your gaze is already at the apex [3]. In a hairpin the apex is far from the braking marker, so you must look early not to lose sight of it.
Milestone: In three laps you turn in later each time and observe the exit.
You drive over a late apex, open the steering and apply throttle as soon as the kart straightens. The textbook procedure looks for the point at which you stay "just barely on the track" and accelerate as hard as possible [1].
Milestone: In five laps you come out without re-steering and raise your exit speed.
In the hairpin the karts are slow and close together. If two karts drive side by side, each must leave at least one kart width to the track limit [2]. Pushing others off is forbidden [2].
Milestone: In the hairpin side by side with another kart you leave enough room without losing speed.
The hairpin is slow but tight, and collisions with other karts are the most common injury mechanism in karting (29.6% in a US analysis of emergency departments) [4].
This guide explains and motivates, it does not replace training with a coach on the track.
Drive the hairpin once deliberately "too slowly" in but get on the throttle as early as possible. The lap time often drops because the exit carries. That is coaching practice; the textbooks describe the same principle with exit speed as the yardstick [1]. Check it by comparing the speed at the end of the following straight, not just the lap time.
On the straight before it, at a fixed marker. The textbook describes progressive, smooth braking [1].
Later than feels right, and then step by step earlier as long as exit speed rises [1].
In the braking zone before a corner a change of direction to prevent an overtaking move is not permitted [2].
If two karts drive side by side, each must leave at least one kart width to the track limit [2].
Yes, with a coach and in small steps. The mistake of "turning in too early" is easy to see here because the exit shows it at once [1].
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