Learn the pinch grip in bouldering: press thumb against fingers, tell wide pinches from narrow ones and let your feet help carry the load. With milestones.
You hold a pinch by placing the thumb on one side of the hold and the fingers on the other. You squeeze the hold together as if with pliers [1]. The honest answer to "Isn't that just thumb strength?": no. You hold a pinch through pressing force, not just pulling.
In a study of 22 advanced climbers, the stronger climbers produced more pull force and more pressing force of the hand on the pinch than the weaker ones [2]. Width matters on top of that: compared with a jug, pull force dropped by 6–19% on the broad pinch and by 31–46% on the narrow one [2]. Convert it: where you pull "100" on a jug, 81 to 94 remain on the broad pinch and only 54 to 69 on the narrow one. On a narrow pinch almost half is missing. That is why more arm strength is not the answer there; better feet and a clean body position are.
Expect broad pinches to feel familiar after a few sessions and narrow ones considerably later (an experience value, not a measurement).
Entry criteria (practice standard):
Both feet stand securely, hips close to the wall, arms stay long. Put the thumb on one side, the fingers on the other and squeeze the hold before you hand over weight.
Milestone: You hold the pinch three times for five seconds each without the hand twisting or slipping.
Build the pressure first, then hand over weight. Grip only as hard as necessary: biomechanics calls this "soft gripping", because the grip force should not be higher than the pull demands [3]. If you crush the pinch, you tire faster.
Milestone: You climb three broad pinches in a row without shaking out in between.
Now movement comes in. Turn your hips so that your weight presses the pinch together instead of pulling it out of your hand. The thumb sets first, the fingers follow.
Milestone: You solve a boulder with two pinches of different widths without a foot slipping.
Because pull force on narrow pinches drops markedly [2], you plan the feet beforehand: higher, more precise, more pressure through the legs. Hold the narrow pinch briefly and unload it as soon as the next hold is secure.
Milestone: You hold a narrow pinch three times for three seconds each with your feet taking weight, then continue under control.
On a slightly overhanging wall the pinch gets harder because the feet need more tension. Stay on the wall with your feet and keep your body under tension.
Milestone: You climb a pinch on a slight overhang cleanly three times without your feet swinging off.
I found no study on injuries specific to the pinch. In general: in a survey of 507 boulderers, almost one in two (44%) was injured within twelve months, and 63% of the injuries affected the upper extremity [6]. Warm up your hands and fingers, ease into narrow pinches and stop at pain in the thumb base, the wrist or the elbow (practice). Keep the landing zone clear and start with moves at jump-down height. This guide explains and motivates but does not replace supervised training.
Do the thumb check before you pull off: set the thumb calmly and let the fingers move toward it, not the other way round. That way you feel instantly whether the pinch "bites" and spare yourself the failed attempt. This is practical experience and not proven.
On a pinch, the thumb lies on one side of the hold and the other fingers on the opposite side. You hold the hold by squeezing it together, like pliers [1]. The pinch is one of the basic grips alongside crimp, pocket, sloper and jug.
Set the thumb early and squeeze the hold before you give weight. Grip only as hard as necessary [3] and keep your arms long. On narrow pinches you lose a lot of pull force, namely 31–46% compared with a jug [2], so your feet work harder.
Usually the pressing force is missing or the thumb sits too late. Chalk matters too: in a small study, nine recreational climbers managed 14.4 pinch repetitions with chalk and 9.1 without [4]. So check thumb position, pressure and chalk, in that order.
Not first. In a study of 32 boulderers, half crimp and front-3-drag together explained 66% of performance [5]. You learn pinches best on the gym wall over many moves; separate training pays off when pinches stop you regularly.
Topout: Climbing Safely Over the Top