Speedbrakes are really mis-named
The name "speedbrake" is really a mis-nomer as implemented on most transport airplanes. Extending the speedbrakes deploys surfaces on the wing that do far more to reduce lift than to increase drag. As such, the flight deck lever would be more appropriately named "lift dump".
As correctly noted in earlier posts above, deploying the speedbrakes reduces lift and requires an increase in AOA. The AOA increase is required to compensate for the associated lift loss. The AOA increase is not required to maintain airspeed, but rather to maintain flight path.
Pilots have long learned that a rapid change in speedbrake position in flight can be rather upsetting to the passengers. At cruise, full speedbrake deployment can result in dumping as much as half of the wing lift and thus rapidly changing normal load factor from the nominal 1g to 0.5g. Speedbrake deployment in air is best done slowly and in coordination with an increase in pitch attitude (i.e., AOA) to balance the associated lift loss. If speedbrakes are deployed at initiation of decent, the AOA increase can be accomplished by simply not pushing the nose over as far as would be needed for a clean wing decent.
Some airplanes include in the control systems automatic compensations to avoid any sharp flight path upset as a result of speedbrake deployment. This feature is provided on Boeing's fly-by-wire airplanes such that speedbrake extension is automatically balanced by the required pitch-up maneuver to maintain normal load factor. Rate limiting the speedbrake command also helps smooth the ride.
It is important to remember that when used on the ground, speedbrakes provide only a minimal increase in drag. The real benefit of speedbrakes on ground is to increase weight on the gear making wheel brakes more effective earlier in the landing or RTO roll.