Honda nighthawk 650 tach stops and runs bad?

Most likely your tach is electrically driven, which operates using a signal from one of the ignition coils. The positive (electrically) side of the ignition system has nothing to do with the turn signals and vice versa. Therefore, I suspect a bad ground connection somewhere on the tach or ignition system. When you turn the direction signals on, the ignition or tach is using the negative side of the direction circuit to ground itself, therefore restoring proper ignition activity. The problem could be a loose connection or a corroded one. Corrosion causes resistance to electricity, resistance results in heat, heat causes increased resistance, which causes even more heat and so on until no current at all can flow through the heated connection. Bad grounds can be very difficult to find, about the only thing you can do is start disconnecting electrical connections (one at a time) and check for corrosion or loosness. There is usually a lot of wire connections behind the headlight, also check under the tank or near the battery. Battery gasing will cause metal corrosion.

You rode through a time-warp and than aliens from parallel dimensions gave you a clue to their existence before returning you and your bike to this time.

Or you have a loose wire electrical short or faulty/loose ground.

If a newer injected model it could even effect fuel delivery.