When it lands. Auto brakes apply to the wheels to target a specific deceleration target. You don’t want to brake too hard and cause undue wear and you don’t want to under brake and miss your taxiway or go off the runway.
Gosh I should have thought of auto-braking. For some reason I kept thinking this was some fancy drone-braking system and couldn't figure out how you'd brake in the air... I never even considered the on-the-ground case. Thanks.