Glide

height is a bank. climbing deposits it, gliding spends it.

hold to climb a cliff at dawn, let go to glide: the shelf across the gap is only reachable from one height, because the reach is the height you spent

A bird on a cliff has two currencies and only one of them is obvious. The feathers are stamina: they pay for going up, they refill only while the feet are down, and gliding costs none of them. The other currency is the height itself, and it is spent the moment you let go, because a glide trades altitude for distance at a fixed rate. That is why the shelf across the gap is reachable from one band of heights and not from any other: too little banked and you fall into the gap, too much and you sail into the wall above it. The rule is lifted whole from A Short Hike, which never explains it either; here the triangle you flew is drawn on the sky once you have landed on the shelf yourself.

Concepts: energy, trajectory, physics.

An interactive you play with one thumb, on Wundr.