Learn the bicycle in bouldering: one foot on top, the other hooking with the toe from below on one hold. With milestones and protection for knee and hamstring.
In the bicycle you squeeze the same hold between both feet: one foot lies on top of the hold (heel or sole), the other hooks with the toe from below. The feet work like the pedals of a bicycle, each side pulls or pushes against the other, and your body hangs in between. The honest answer to "Isn't that just a heel hook plus a toe hook?": almost, but only the working against each other makes the bicycle. Picture two hands squeezing a ball, except they are feet and the ball is a hold.
The international federation IFSC describes both building blocks: the toe hook holds the position and prevents swinging backwards, the heel hook carries body weight and makes the next move easier [1]. Biomechanics explains why it works: if a foothold points away from the body, force transfer works best by pulling with the heel or toes [2].
Honestly up front: I found no study that examines the bicycle itself. What follows rests on the two building blocks, heel hook and toe hook, and on practice. Expect to have both building blocks clean separately after a few sessions, but the interplay takes weeks (an experience value, not a measurement).
Entry criteria (practice standard):
Hook the toe from below around a hold and pull up without bouncing. The toe hook holds you in position [1].
Milestone: You hold a toe hook three times for five seconds each without it slipping.
Lay the heel over the hold from above and pull. The heel hook carries body weight [1], so you build it with little load.
Milestone: You hold the heel hook three times for five seconds each without your hamstrings twitching.
Find a wide hold or a volume. One foot from above, the other from below. First at jump-down height, with your hands on comfortable holds.
Milestone: Both feet lie on the hold, and the hips hang calmly in between.
Now you tension the feet against each other: the upper foot pulls in, the lower one hooks. The hips come to the hold, the center of mass stays over the base of support, an advantage of the heel hook [3].
Milestone: You hold the position three times for three seconds each and release one hand.
Build the bicycle into a boulder where it opens a move. Plan beforehand where you release it and which foot you continue with. Be clear whether you need the bicycle as a rest or as a start for a move, because that decides how much counter-pressure you build.
Milestone: You use the bicycle in a boulder and release it under control.
The bicycle contains a heel hook, and the heel hook was the most frequent mechanism in a case series of 77 acute knee injuries (40.3%); as a specific climbing injury the study describes irritation of the iliotibial band during the heel hook [4]. According to a review, acute hamstring tears are often caused by the heel hook [3]. There are no figures on the bicycle itself. Build-up instead of shortcuts: warm up hips and hamstrings, start at jump-down height, release slowly and stop at pain in the knee, hip or back of the thigh. This guide explains and motivates but does not replace supervised training.
Do the pedal test: tension both feet alternately and then let go of both at the same time. If the hips hardly wobble, the bicycle really carries you. This is practical experience and not proven, but you notice immediately whether the hold carries you or only holds you.
In the bicycle you squeeze a hold between both feet: one foot from above, the other with the toe from below. Both foot contacts stabilize the body so you can release a hand. It combines heel hook and toe hook [1].
When the boulder overhangs and a wide hold or volume lies between the feet. It pays off if it frees a hand or brings the hips closer to the wall. Without the right hold position it does nothing.
It contains a heel hook, and that was the most frequent mechanism in a case series of acute knee injuries [4]. Acute hamstring tears are often caused by heel hooks [3]. There are no figures on the bicycle itself.
Practise toe hook and heel hook separately, then both on the same hold at jump-down height. After that you tension the feet against each other and release a hand. Only then does the bicycle go into a boulder.
Double Dyno: Both Hands Jump at the Same Time