The left magnet activates when you shoot the Orbits repeatedly to LOCK the first and second balls. That's the only time I can think of.
I have a weak plunger as well. The auto-kicker's alignment and the springs in the plunger have a lot to do with it. I replaced mine and now they're quite strong...but the alignment is causing the ball to plunge to the left slightly, slowing it down.
I replaced the whole thing for less than $10 dollars. It's really not that expensive to do. I took some tips from Gameroom Collectibles on youtube to learn a bit on how it all went together and the importance of the right parts. Take a look at the video here:
(Just shot a 216M on it. I've never cracked 300M.)