Dial calipers have a rack which meshes with a gear that drives the pointer. Sometimes the gears jump a tooth or more, resulting in the pointer not reading zero when the jaws are shut. I am guessing, here, but the tool may be for tipping the spring loaded gear out of mesh with the rack, to get the pointer reset. I had a Tesa dial caliper, but that was 40 years ago, and my memory is not great.

Somebody will be able to post some photos showing exactly how its done.

I thought the things must've stopped doing that since then, otherwise why are people still buying them?

Hi I have a dial caliper and in the box is a copper coloured strip which is used to adjust the dial if it jumps a tooth. You slide it between the rack and gear and then slide the carriage along a small amount and then pull out strip it may take a few tries to get it right.

David

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Just one small caveat, if I may: The descriptions of the tool's purpose are fine, but it is always worth checking the rack and cleaning if appropriate. … Said foreign body can sometimes be stuck in place rather tenaciously

MichaelG.

This is the weakness of a design with exposed gears, you have to take care not to drop then in the dirt.