Hard tack rolling pin. Janne / Pixabay.com / 2017 / CC0 1.0
A hard tack rolling pin is a special rolling pin used for rolling out unleavened bread, especially hardtack and Scandinavian flat breads.
It has evenly but closely-spaced dull short spikes carved into the wood which make indents in the dough.
The indents help prevent the dough from rising in various places, making it easier to break apart into pieces when cooked. It also gives a rough, cooked surface texture.
The dough is rolled out first with a regular rolling pin, then finished off with a pass from this rolling pin.
Substitutes
Poke surface of rolled-out dough evenly with the tines of a fork.