Sodium acetate is the chemical in the reusable handwarmers.

You can coat a reactive chemical in a protective coating, then like popping a balloon, the pressure breaks the coating and triggers the reaction. The easiest example of this is putting an oxidizer and a reductant in the same bag.

There are lots of exothermic crystallization reactions where the pressure nucleates a crystal in supersaturated solution. You can control that pressure using engineering of the container, but it's not unique to the actual chemical.