FAST XFI driveshaft speed sensor kits measure driveshaft rpm to provide more accurate data for the traction control unit. Two magnets are fitted to the collar around the pinion yoke, which attaches to the driveshaft. When one of the magnets passes the pickup, it sends a signal to the FAST XFI unit. The FAST XFI then uses that signal to calculate both driveshaft rpm and driveshaft acceleration. As a result, these speed sensor kits provide much more precise readings, as opposed to determining driveshaft speed solely through variable engine rpm. The data they provide is perfect for drag race applications.