If you take that R6, coming around that bend at say 10,000 RPM, the crank spins at 166 revolutions in one second. So in that one second, each piston goes from a dead stop to about two inches, comes to another dead stop, and accelerates madly to a dead stop again 166 times. Incredible.
But the valves are also moving madly letting in gas, closing, and exhausting. During those 166 revolutions, the spark plugs are sparking every time the piston comes up near TDC. Wait, that means the spark plug is not only firing on the compression stroke, it's also firing when a piston is compressing the exhaust out the exhaust pipe. That means that if there is any unburned fuel (running rich) in the exhaust it will burn.
Not that I've ever done it, but have you ever changed spark plug wires on a V8 and got them on wrong. Weird things can happen like a spark in a cylinder when the intake valve is open. Flame up through the carb. A spark when the exhaust valve is open won't be bad unless it's running real rich, when a spark can blow off the muffler. Don't ask how I know this.