For engines with an impulse coupling on the left mag (the thing that emits the loud click when you pull the prop over by hand) OR has a "shower of sparks" on the left mag:
Then if the a/c has a combined mags on/key start it will almost certainly ground out the right mag when the key is at the START position. Releasing the pressure on the key returns the key position to BOTH and both mags should now be live.
With the same scenario in para 1. IF the a/c is equipped with independent mag switches and an independent start switch, you should only start with the LEFT mag selected on.
If hand swinging the previous paragraph applies, too.
Hope that this helps,
Stik