There are two things happening, the movement of the SPEED BRAKE LEVER and the actuation of the panels, which (SYSTEM A) are split into two groups, GROUND and FLIGHT.
When you have wheel spin up on any two main wheels, the lever moves UP and the FLIGHT panels extend. Once there is compression on the RIGHT MAIN GEAR squat switch, the GROUND panels extend. If there is no wheel spin up detected, the system will delay deployment until one of the squat switches is compressed.
The advantage is an earlier deployment of the boards during landing, as the wheels will generally spin up before the squat switches activate.
The bypass of the wheel spin up is a belt and braces approach, enabling two protections: allowing spoilers to extend even with an anti-skid failure which processes the wheel spin inputs, and preventing the GROUND spoilers from deploying while airborne.