Apparently sodium acetate and water. They aren't too clear on the details, but from the clue that it's a crystallizing reaction, and that when boiling it you have to make sure that all the crystals are re-dssolved before letting it cool down, I can make a good guess. First, they picked something that would have an exothermic crystallization reaction, so that's how it produces heat. It's a supersaturated solution, and so it's ready to begin crystallizing at any moment. But any reaction like that needs some impurity, or rough spot, or something to act as a site to start crystallizing (this is the same reason that raindrops and snowflakes form around dust specks and the like, or you can make rock candy by dipping sticks in a supersaturated sugar solution). So pressing the button must do something that introduces a crystallization site, but without an example, I can't think of what.

However, I think I may need to get some of those. They look pretty cool!

[edit to add]
It looks like I was more or less correct. Here's some more information, and here's a patent on the activator (or at least a similar one).

__________________
Conserve energy. Commute with the Hamiltonian.