Singer tells you this in (some of) their manuals.
|From Singer 201K manual|
How they Work
These controllers use a stack of thin carbon discs which are compressed by the button you step on to allow current to flow. A friend of mine in Canada has a blog which explains this in detail: http://www.archaicarcane.com/singerattheraces/
Why Unplug?There are three good reasons I can think of to unplug your machines:
1. Over time current starts to leak and this can make them hot. This heat can build up enough to start a fire, which can happen when you aren't home or when you're asleep in bed.
2. The higher voltage ones (220V/240V) were fitted with a capacitor to prevent AM radio interference. When this breaks, it short circuits and the machine suddenly goes at full speed. Yes it can happen when you're not at home!
3. Your machines are fascinating to kids, who love to play with them. If the machine is plugged in, kids are more likely to be injured. When my machine isn't in use I unthread it, place a piece of fabric under the foot, foot down and needle down.
You can certainly tune bakelite controllers so there's less danger, but the best thing you can do is to get into the habit of unplugging them when you're finished sewing. Don't be frightened of these. They're original to the machines and perform very well, so there's no reason to get rid of them. Knowing how they work is valuable.
A few years back I did an entry on fixing them up. Not as comprehensive as archaicarchane's, just removal of the capacitor.
What do they look like again?Not all carbon disc controllers look like this:
|Singer Bakelite Foot Controller|
Some of them look like this:
|Husqvarna controller from early 1970s|
I was surprised they all didn't use coiled (resistance) wire, but they don't.
That's all. You've been advised to do it and told why, the rest is up to you. Have fun!