A child watches an empty blue bicycle roll across a park path while another bicycle rests nearby. something needs explaining

Why does a moving bike stay up?

A riderless bicycle can roll upright, yet the same bicycle topples when stopped. What catches a lean?

To rescue a leaning tower, move its support back beneath its weight.

A child slides a small board beneath a leaning stack of blocks before it can topple.

Tap the base. Moving support beneath the leaning top can stop the fall.

Set the starting speed of a supported bicycle. No lean has been released yet.

Inspect the steering joint while the bicycle is held in place. A locked joint cannot turn; a free joint can turn. We have not released a lean yet.

Two fast bicycles get the same rightward lean. One steering joint moves; one is tied straight.

Same fast roll. Same lean. Only the steering joint changes.

Which bicycle can catch the lean?