Far from a silly question....
Auto brake is armed on gound for take off and is only in the active range above 72 knots on take off role, it is activated on retardin of the Thrust levers to idle which induces spoiler deflection in turn activates autobrake MAX. Now for landing it is very simular in that on the deflection of spoliers there is a timed delay on fixed decel rates for MED and LOW. Med is 3 meters per second squared after a 2 second delay at touch down whereas LOW is 1.7 mps sq. with a 4 second delay (if my memory serves me correctly). They are activated on SPOLIER deflection or if spoliers were not armed then on the selection of reverse.
We also see DECEL in green on the autobrake selectore when 80% of the selcted braking required is achived.
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Gravity always wins....