It is not just that simple . The big 3A diode D2 is doing that job. The trace at the on off circuit might burn as shown in this thread.
Unfortunately the pictures in the answering thread is gone now, due to the shift to another server for discourse.
But tooki57 had picture of the trace fully burnt away of shortcut going from small D3 diode through on off switch to GND. (When positive is at GND)
And the relay is indifferent to what direction power is going, and will activate.
The D3 is a flyback diode to take energy from spool in relay when you switch off.