[David G4EBT]

If there's a break in the track, as shown in the diagram below, there will be an open circuit between tags 1 & 3, (instead of 500k Ohms). If the break is between tags 1 & 2 (2 being the wiper), there will be an open circuit between those tags. Between tags 2 & 3, there will be a resistance which will vary as the potentiometer is rotated (leftwards in the diagram), until the wiper of the pot passes over the break, when the meter will show an open circuit.