Learn the crossover in bouldering: reach past or under the other hand, turn your hips in and protect shoulder and elbow. Step by step with milestones.
A crossover means you reach past or under the other hand to the next hold instead of matching. The honest answer to "Why cross if I can match?": because a crossover reaches a hold you cannot grab after a match, and because it saves time on the hold. Whoever only matches gives away reach.
Sports science knows hand crossing both frontal and rotated: depending on whether your hips stay parallel to the wall or turn in, you cross in a different way [1]. The crossover follows the three-phase model of a move: first shift the center of mass, then move the hand, then stabilize [1]. A lab study with nine elite climbers shows that experts actively work with weight here: they let the center of mass sway sideways regularly, which went along with a changing weight distribution between the limbs [2]. So you don't have to stay rigid; you may deliberately send your hips into the movement.
There are two variants: when crossing over the holding arm, the free hand passes above; when crossing under, it passes below. Which one fits depends on where the next hold sits and how your body stands relative to the wall. Both have this in common: the center of mass moves first, the hand follows.
Expect a clean crossover to take a few weeks of practice (an experience value, not a measurement).
Entry criteria (practice standard):
Hold a hold with a straight arm, turn the hip on the holding side toward the wall and shift your weight onto the opposite foot. This is the preparation phase [1].
Milestone: You stand ten seconds in this position without your body swinging open.
Guide the free hand over the holding arm to the next hold. Don't jerk; let the hand travel to where it can grab securely.
Milestone: Five clean crossovers per side without the standing foot slipping.
Guide the free hand under the holding arm. The shoulder stays low, the elbow points down rather than outward.
Milestone: Five clean crossovers per side with a low shoulder.
After grabbing, you need to set the hips so the new position is stable. Otherwise your body swings open like a barn door [1].
Milestone: Three crossovers in a row without a barn door.
Decide consciously on each move: crossover or match? If the match doesn't get you further, cross. If the match is enough, skip the crossover.
Milestone: You climb a medium boulder and use two deliberate crossovers.
In a crossover one arm often works in a bent position while the other reaches over or under it. In a lab study with 17 recreational climbers, elbow flexion moments rose with increasing elbow flexion, and the moments at the shoulder (flexion, adduction, rotation) were also higher with bent arms than with straight ones [3]. The study looked at hangboard hangs, not crossovers, but it shows that bent arms demand more from shoulder and elbow. A systematic review on overuse injuries found that a strength training intervention prevented shoulder and elbow injuries [4]. Warm up, don't cross cold and start at jump-down height. This guide explains and motivates but does not replace supervised training.
Send the hips first, the hand second: if your hips already point toward the next hold, the arm hardly needs any distance. This is practical experience and not proven, but for most people it saves part of the strength in the crossing arm right away.
A crossover is a move where you reach past or under the other hand to the next hold instead of matching. It gives you reach you would not have with a match [1].
Cross when the next hold is on the other side of your body and you can't reach it after a match. If the match is enough, skip the crossover: it demands more coordination.
When crossing over the holding arm, the free hand passes above; when crossing under, it passes below. Which variant fits better depends on where the next hold sits and how your body stands relative to the wall. In both cases you shift your weight first.
Bent arms load elbow and shoulder more than straight ones [3]. Keep the holding arm as long as possible, warm up and increase the difficulty slowly.
Usually the hips stay frontal or the weight isn't over the standing foot. Turn the hips toward the wall and shift your weight before grabbing. That's how you avoid the "barn door" [1].
Foot Swap: Switching Feet on One Foothold