Test light on a running pump

I just watched the Sante Fe fuel pump video.  At the end of the video, Paul said that a test light will not light when checking a running pump because as the pump is working, it creates resistance and won't light the test light.  That is confusing to me.  There is plenty of current in the circuit to light the light 6-8 amps.  There is a good power, good ground, and current flow.  Can you expand on this? I'm sure it has something to do with CEMF.  But it still is confusing. Thank you!

Can you post a link to the video, please?

I am assuming that Paul had his test light connected to Battery Positive (+), and was touching the power feed to the fuel pump. 

If the fuel pump power feed wire is NOT energized, there will be no voltage on that wire.  However, there WILL be a ground that comes THROUGH  the fuel pump, from the fuel pump ground wire. A test-light connected to Battery (+) will find this ground THROUGH the fuel pump, and the test light will light because it has Battery voltage on one end, and a ground (coming from the fuel pump ground wire, THROUGH the fuel pump), on the other.  Power + Ground = Light

Now, if you energize the fuel pump power feed, it will have battery voltage. If the test light is connected to battery (+), at the battery, and you touch the point of the test light to a wire that also has battery (+), the test light will not light because there is no longer a ground, (and no difference in potential). Your test light will have battery voltage on both ends. Power + Power = No Light

 
I apologize for the crude/bad drawing. I suck at drawing with a pencil, and am even worse doing it with a mouse.