What did slave masters do if their slaves weren't working? The slave masters turned them off and turned them back on.
I think that's is quite insensitive, but I don't fully understand what you're trying to say about turning them on and off.
It's a joke about slave masters turning their slaves on and off.
Slavery is a very cruel part of American history, and it can be very distasteful to still joke about some things. Black people fought hard to be free and get be treated equally, they are human just as you. If you keep trying to joke around like this, many people will get hurt and you'll ruin their self-worth.