Seeking wiring power explanation

I was installing a transmitter for the brake light in my helmet (Gmax GM54S) and was confused by the power going to the yellow wire (brake wire) under the pillion seat. I had expected that the voltmeter would detect power through the yellow wire only when the brakes were applied. However, it was just the opposite. It detected power in the wire without the brakes applied and the no power as soon as brakes were applied.

Can anyone tell me why that would be?

EffinJR,

You can make a brake light turn on by sending power through the brake switch, to the lamps. You can also apply power to the lamps at all times, and send ground to the brake lamps from the brake switch, or in this case, sending ground to the brake light relay. Either way works. By using the brake switch to ground a relay, the brake switch will last a long, long time, compared to sending the full lamp power through the switch. A relay makes a very low current draw, through the brake switch.

Infrared

Infrared,

Makes sense, thanks for the explanation!