Learn the half crimp: check your finger angles, hold edges cleanly, choose between open hand and crimp and keep your pulleys out of trouble. With a progression.
In the half crimp the middle joint of your fingers is bent at roughly a right angle, the end joint is not hyperextended and the thumb stays free. It is the standard grip for edges that are too small for the open hand. The honest answer to "half crimp or open hand?": as open as possible, as closed as necessary.
The reason lies in the pulleys, the small bands that hold the flexor tendons against the finger bone. In a biomechanical model the full crimp (middle joint bent, end joint hyperextended) loaded the A2 pulley 36 times more than the open slope grip, and the A4 four times more [1]. How the half crimp sits in between was not measured on its own for the pulleys in the studies we read. It is a classification, not a measurement.
How much force you have on an edge depends on grip and depth: in a study of ten advanced climbers, maximal vertical force ranged from 351 to 576 newtons depending on grip and edge depth (1 to 4 centimetres), roughly 36 to 59 kilograms, and rose with depth [2].
Honest classification: you set the angle in one session. Holding it under load when the edge gets small takes much longer. Expect several weeks (experience value, not proven).
Entry criteria (practice standard):
Put your fingers on a table edge. The middle joint sits at about a right angle, the end joint is relaxed, not pressed back, and the thumb stays free. Take a side-on photo. Compare it with the open grip (middle joint almost straight) and the full crimp (end joint hyperextended, thumb over the top).
Milestone: on a photo you can tell whether you are gripping open, half or full.
Both hands on a deep edge, feet on the ground, so you carry only part of your weight. Hold the angle from step 1 for five seconds.
Milestone: you hold 3 times 5 seconds without the angle collapsing.
Find an easy edge boulder with deep edges. Move slowly, keep the arms long and check with your phone that the angle holds in the hardest move.
Milestone: one full easy edge boulder where the angle looks right on video in every move.
Force drops with depth [2], and technique drops with it. Go only one step smaller per session and stay on that step until the angle holds.
Milestone: you hold the smaller edge without the end joint slipping or the thumb starting to cramp.
Choose the grip by the edge, not by habit. If the open hand is enough, stay open. The half crimp comes in when the hold gets too small.
Milestone: on a boulder with mixed holds you decide consciously on every move.
In the crimp the flexor tendon "bowstrings": it lifts off the bone and presses against the A2 pulley. In a measurement on four people, the load over the A2 was about three times the force at the fingertip, up to 116 newtons (about 12 kilograms) [4]. During the warm-up the distance of the tendon over the pulley edge grew by 0.6 millimetres, or 30 percent [4]. That does not prove that warming up prevents pulley ruptures, but the tendon position changes, which is why easy routes come before small edges (practice standard).
Tape is not a shield: a review found low to moderate evidence that taping might reduce tendon lift-off by 15 to 22 percent, but no effect on maximal strength in healthy climbers [5].
If you feel sudden pain at the finger base, stop immediately and have it checked by a doctor. Severity decides the treatment: mild injuries conservatively, severe ones often surgically [6].
If the edge slips away, you fall. Keep the landing zone clear and land feet first with soft knees. This does not replace supervised training.
Film your hand in the hardest move from the side, not while standing. Standing, you see yourself the way you would like to grip. In the move you see what the fingers really do: whether the end joint tips, whether the hand re-grips, whether the thumb cramps. Twenty seconds of video say more than twenty minutes of feeling.
A half crimp is a finger grip in which the middle joint is bent at roughly a right angle, the end joint is not hyperextended and the thumb stays free. You use it for edges too small for the open hand. It sits between the open hand and the full crimp.
It depends on the hold. On holds where the open hand is enough, it is the gentler choice: in a model, the load on the A2 pulley was 36 times lower in the open grip than in the full crimp [1]. The half crimp comes in when the edge gets too small. As open as possible, as closed as necessary.
The studies we read cannot say for sure: they measured the open grip and the full crimp, not the half crimp. What is certain is that the crimp loads the pulleys clearly more than the open grip [1]. So warm up, progress in small steps and stop if you feel pain at the finger base.
Check your angle with a side-on photo first, then hold deep edges cleanly and go smaller step by step. Force drops with depth [2], so patience is part of the technique. Switch between open hand and half crimp depending on the hold and keep your arms long.
Holding Slopers: Friction Instead of an Edge