Learn the outside edge in bouldering: turn your hips in, step with the outer shoe side and reach further with less arm strength. With milestones.
The outside edge is the side of your shoe where the little toe sits, and it belongs to turning in: you rotate one side of your body toward the wall and step with the outer side of the shoe. The honest answer to "Why should I turn in at all?": because you reach further and hang less from your arms. Anyone who only climbs face-on gives away reach and strength.
The German Alpine Club (DAV) describes turning in in its bouldering badge booklet like this: you turn one side of your body toward the wall, which lets you reach further up or sideways, gives you more power from your feet and needs less strength in arms and shoulders [1]. In sports science this belongs to the "rotated" techniques: your hips are no longer parallel to the wall, and foot contact runs over the outside edge of the climbing shoe [2]. This creates a problem everyone knows who has ever swung open like a door: the "barn door". The free leg counteracts the opening and helps to bring the hip toward the wall [2].
Expect it to take a few sessions until you no longer have to "think" about turning in (an experience value, not a measurement).
Entry criteria (practice standard):
Stand at a vertical wall, hands on big holds. Rotate one hip toward the wall and the other away from it. The foot on the side turning toward the wall stands on the outer side of the shoe.
Milestone: You stand ten seconds on the sideways-turned foot without the shoe moving.
Find a small foothold, put the outer side of the shoe against the wall and press. Your hips follow the rotation. Important: don't let the foot dangle, give it pressure.
Milestone: Five clean attempts per side where the shoe stays put.
When you turn in, your body wants to rotate away from the wall. The free leg counteracts [2]. You can brace it against the wall or carry it next to the standing leg.
Milestone: You let go with one hand for three seconds without your body swinging open.
Turn in and grab a hold you cannot reach face-on. That is the whole point of the exercise [1].
Milestone: You reach a hold that is one hold further than face-on.
After the move your hips turn back toward the wall and you stand stable [2]. If you don't turn back, you get stuck in the rotation and lose the next move.
Milestone: You climb an easy boulder and deliberately turn in and back at least twice.
Turning in loads knee and hip differently than standing face-on. In a case series of 71 patients with 77 acute knee injuries in climbers and boulderers, four mechanisms stood out: heel hook (40 percent), ground fall (22 percent), high step (21 percent) and drop knee (17 percent) [3]. A case series says nothing about risk per move, but it shows where knees suffer. So turn in only as far as the knee stays over the foot, and don't force a rotation cold. Start with easy boulders at jump-down height. This guide explains and motivates but does not replace supervised training.
Check your knee: if it points from the foothold toward the wall without collapsing, the rotation is right. If it points inward, turn in less. This is practical experience and not proven, but it is a quick self-test you can do on any foothold.
The outside edge is the side of the climbing shoe where the little toe sits. You use it when turning in, with one hip toward the wall. The technique increases your reach and relieves arms and shoulders [1].
Whenever the next hold is to the side or far up and you cannot reach it face-on. Turning also helps with sidepulls, because it positions your body against the pull [2].
A barn door is your body swinging open away from the wall when you reach sideways and your center of mass lies outside the line connecting your contact points [2]. Turning in and a stabilizing free leg are the remedies.
It loads the knees differently than standing face-on. Knee injuries in boulderers are documented, mostly with heel hook, high step and drop knee [3]. Turn in only as far as your knee stays over the foot, and build up slowly.
Sidepull: Pull Sideways and Push Back With Your Feet