These hand warmers are just resistors, they heat up with the resistance to the current flow through them. Nothing special about them, just a big resistor.
As already mentioned though you have to have enough power to run them. I know guys that have them on their quads and trikes and they have to charge the battery every time they park it because they were over budget on amps running the warmers.
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1984 Honda ATC200ES "Big Red"
1982 ATC200E "Hondie"
1988 TRX300FW "Project Quad" Still in progress....