OK, vibrator advice, anyone? No, no, stop sniggering........

[The Inspector]

It provides enhanced spark, usually to one cylinder via the magneto coil, while the engine is cranking and the coil may not be putting out enough voltage to fire the plug efficiently enough to set off the fuel/air charge, also sometimes referred to as a Booster coil on lower powered/small aircraft

[Rick65]

Thanks for the information. The formal name is much more explanatory (but boring).
Is it called a vibrator because the motor doesn't initially start on all cylinders?

[The Inspector]

The starter has a very tough job. It has to get the entire reciprocating assembly working from a less than optimal leverage location in the accessory case of the engine. Mechanically, given enough gearing you could pull a freight train with a wrist watch, but practicality gets in the way, so if you can get one cylinder to help the starter to get things moving that's a big time plus. Magnetos produce more voltage as their rotational speed increases so down low, not much voltage or amperage is available to make a weak spark jump the gap to ground and set off a cold, wet charge of fuel/air in a cold cylinder particularly if the carbon based unit in the seat is a believer in 'more primer strokes is never enough' (same school as 'it's snowy and slick out so I'll give the car more gas').
The mags fire the plugs in order but again give a very weak spark (if you touch a lead you may not buy into that, but you can kill yourself with a 'D' cell battery, it's amperage not voltage and about 1.8 Amps is enough) so anything you can do to help the engine so much the better.
You are taught as an engine mech, to touch the impulse lead for a turbine engine to the case when removing an ignitor to ground it out. A co-worker had done this routinely for years, and just that one time, the booster was holding a full charge and as he touched the lead to the tailpipe shroud he got a huge intensely white spark, when he turned around he had the strangest startled look on his face, all he said was 'Now I understand..'

TRIVIA ALERT the mags on a T/FD dragster or F/C pump out 80 Amps to fire the charge that makes 8000 H.P.

[therealJR]

Its also fits the Douglas A26

[51fixer]

When I start the Merlin in a P-51 I start the engine with the mag switch off.
Yup, it'll roar to life without the magnetos on line.
That it because the C-1 Booster Coil provides a continuos stream of energy.
It is connected to the L/H Mag and inside the mag it provides a path to the rotating Distributor which actually has 2 sets of fingers on the distributor.
The normal and a retarded finger. The normal one provides the spark while the engine runs and is timed to whatever timing is setup. Original it was 45 degrees advance (after TDC) but today we run them around 38 and 42. But those aren't firing yet as the mag switch is off and these mags are grounded at the moment.
I mentioned a retarded finger and that is a finger in the distributor that runs the spark at a point closer to TDC. The Booster coil gets power from the starter circuit so when the starter is engaged, the booster coil makes sparks and they get sent to each spark plug on the exhaust side of the heads which is the plugs fired by the L/H mag.
That is how the engine will fire and start to run without the mag switch turned on.

Help in starting on US engines comes in 2 ways- Some add current to a mag when the engine is turning slow at start up to make the spark hotter. This methods is Exciting the Mag.
The other method is as I described where an artificial spark is produced outside the normal path and operation of the mag and is distributed by a retarded portion of the distribution mechanism in the mags.

[airnutz]

Man, I do miss the Inspector. What a great asset he was...a retired guy with his experience who roosted here on almost a daily/hourly basis. He also had a pretty good aviation reference library handy as well. He could be a bit surly at times, but he was our surly old dude! Luckily we still have Rich, Taigh, and others, but it'll probably be quite a while before any of 'em retires or can spend that kind of time with us...