@sow thanks for your excellent explanation
each landing the reversers are used first and speedbrakes activation is triggered by the reversers.
... just to be precise... in all second generation Boeing airplanes ...
during landing ... if in ARM position the speedbrake lever is driven aft to the up position when both Thrust Levers at idle and the main landing gear is fully on ground (not tilted)...
... on the ground when either reverse thrust lever is moved to reverse idle detent, the speedbrake automatically extend. The speedbrake lever does NOT need to be in the ARMED position.
Automatic speedbrake are NOT available in the secondary and direct modes. This is why in some old 777 QRH it's possible to find manual speedbrake deployment required on RTO ....