Yes, it's magnetized, I believe they key has to be on to do this, if you didn't have it on, try it again. Your not really "hitting" the thing with a screwdriver, you're taking the "flat" part of the end of the screwdriver and moving it towards the tip of the pickup and then back, rather quickly. It's OK if you tap the pickup, but the point is by moving a metal object such as this in front of the pickup, you are simulating the movement of the exciter in the tailshaft. If the speedo needle jumps, you then know the problem is with the piece in the tailshaft. You have to be sure to have someone watch the needle when you are doing this.
Gilly