Over-drive, stated as a percentage, is the extra Ampere-turn applied to a reed contact after closure, before measurement of Contact Resistance. This is done to fix the direction of the remanent flux.
Contact Resistance of a reed contact is usually measured at 25% overdrive.
Overdrive, when using a magnet is achieved by moving the magnet closer than the actuation point. Using a Reed Switch in an application with a higher Over-drive gives a longer life.