Remote Start for my Dust Extractor
The fifth connection is under-voltage lock-out - notice the symbol "U&lt;" in the little box on the switch: -



It also tells you on page 1 of the data sheet that there is a feature that prevents restarts should the AC fail. That connection is associated with that feature and, maybe the connection to the motor is because there is an over-temperature contact in the motor that can also trip the magnetic relay internally.

This makes adding a remote start/stop circuit somewhat problematic but, if you are fit for the task, the new start/stop circuit would need to come down stream of the KJD17B and could be the conventional self-latching relay circuit and wouldn't need to be double pole: -