There is a rail, another rail that turns, a detector and a powered rail, and on the opposite side of the turning rail (no detector) the detector triggers and causes the turning rail to switch when that is NOT the desired behavior.

Reproduce:

Mine cart direction ->

reg-rail   reg-rail   turn-rail-> detector    powered 

                              reg-rail

Without the detector the minecart rolls over the bolded rails pieces and keeps going straight.  With the detector, the turn-rail switches before the minecart reaches the detector rail, causing the cart to turn when it shouldn't.