@PD. The stock ignition coil has its backside ground wire running out the back of the stator plate and up the frame, through both brake levers to a grounding lug in the headlight bucket. There is also a black wire running parallel off this ground wire going directly back to the brake light bulb.
Squeeze either of the two brake levers and the ground path is broken. The coil then searches for the path of least resistance to a ground which is now the brake light bulb which now magically lights.
This is also the supposed safety feature you mentioned. If the brake bulb is burned out the coil can find no ground and the spark stops till you let go of the lever resuming the ground.