injectors and noid lights

ok here is a copy of testing injectors with a noid light without removing the plenum. follow the injector wires over to the harness connector and test from there. my question from here, as i have never used a noid light is, do the contacts on the back of the light have to touch a bare contact or wire or does it pickup an electical signal through induction if you know what i'm trying to say. thanks your help is greatly appreciated

You need direct continuity for them to work.

thank you