Small Engines (Lawn Mowers, etc.): Coil kill wire, jr dragster, racing engines

Question
Is the small wire attached to the back of a 5hp Briggs coil a kill wire?  My son's brand new Jr. Dragster engine runs great but will not shut off when I ground this wire.  Tech support at the company who built the motor tells me the small wire is for a tach.  Thay want to sell me a $90.00 manual kill switch.  If indeed it is a kill wire why won't the engine stop when I ground it?

Thanks.

Answer
If you ground the small wire on the back of the coil does the engine stop?

If the coil is stock, then the answer you are looking for is yes.

If the coil is an after market performance coil then I would have to know who made the coil to know for sure.

Most racing engines have a spark plug boot with and integrated handle for stopping the engine; you just pull the rubber are and dis-connect the spark plug.

Let me know what you find out.
Eric