How is power to an entire room restored by plugging something into an outlet in an adjacent room?

We lost power to an entire room about a week ago. All outlets and ceiling light are dead and no breakers were tripped. Unplugged everything in the room, reset all breakers anyway and checked the only GFCI in the house which is downstairs from the room in the kitchen. The GFCI outlet was not tripped but I still did the test and reset. Nothing.
Scheduled appointment with electrician who will come next week.

Today I needed to use my printer which was in the dead room. Took it to the bedroom across the hall and plugged it into an unused outlet in the room. The light came on in the dead room, I checked all the outlets and they are working as well. Unplugged the printer and power to the room went out again.
What would cause this and would it be safe to keep something plugged into that outlet until my electrician comes? Or could that cause more damage?

1. the outlet that restored power is directly below the light switch in the working bedroom but does not require the switch to be on to work. It is the only outlet that restores power to the dead room
2. when I plugged in the printer there is a slight flash (arc?) in the outlet
3. the dead room and the working room closets are back to back. The restoring outlet is on the same wall as the closet