Double-tap the sky
What it is. Two quick taps on empty sky — your home screen — run one bound action.
How to do it. Tap twice, fast, on an open part of the sky. Each touch has to be a quick tap, not a hold, and the second has to land close to the first and soon after it. Too slow or too far apart and it reads as two single taps instead. How soon the second tap must follow is the "double-tap window (ms)" slider (200–700, 340 by default) in Settings › accessibility, under "extended · timing".
Default. lock. The double-tap can be rebound to any app, folder, shortcut, or system action; see Assign an action to a gesture. You'll find its picker, "◎◎ double-tap on the sky", in Settings › gestures.
See also. Assign an action to a gesture · Hold to bloom the ring · Gestures conflict with system gestures