Just to add a different twist...
On a glider the Air Brakes fitted to the wing operate slightly differently. They still reduce lift and increase drag but aren't normally used to the control speed or perhaps I should say they aren't directly used to control speed. Typically on approach the speed is set using elevator then the air brake is used to adjust the glide angle.
In an ideal landing the glider would be flown (using good planning) into a position where about half air brake can be used all the way down final. If you find you are using all or none then basically you got it wrong. If you find you need all the brake it means you started too high or over estimated the wind speed - and you risk an overshoot. If you find you need no brake then you were too low or you under estimated the wind - and risk an undershoot.