Here's a couple of pictures that show how the o-ring works to hold the throttle on a Concours C10.
The idea is to roll the o-ring into the groove between the throttle and the bar end weight to allow the friction of the o-ring to hold the throttle in position. This allows you to shake out the cramps, or get the circulation going, in your right hand on long trips.
The first picture shows the o-ring in the "off" position. The second shows it "on". If you look closely at the ribs on the throttle grip you can see I've twisted the throttle in the second picture and it's being held in place by the o-ring.
I suppose you might find a generic o-ring or even a thick rubber band, that would do this - but people claim this silicone Caterpillar o-ring is just the right stickiness and width.
I bought mine from Tornado Heavy Equipment Parts using eBay. Two o-rings plus shipping were $4.93 and took 4 days to arrive. Jet Gasket and Seal is selling them on eBay for $8.29 for ten o-rings with free shipping.