voyager overdrives with 1/4" mono plugged into R out

I dont understand why this works?

Without the other end plugged into the EXT in, it overdrives as soon as I plug a mono 1/4" cable into the RIGHT output?

I would have assumed I needed the other end plugged into the EXT in, to get that effect?

Also my mind may be playing tricks on me, but it appears to sound/react differently once I do plug the other end into the EXT in

It is subtle though

Thanks!

Ps I am using the mono LEFT output as audio,