Major soldiers can hunker down and make nearby allies take less damage when they are fighting underground. This ability is used continuously until the target is eliminated.
Exploding
You can disable exploding to prevent your minor soldiers from exploding and let them attack normally.
You can enable both exploding and attacking to make minor soldiers explode immediately against any enemies.
You can enable exploding and disable attacking to make minor soldiers explode only against attacking enemies.
In real life, exploding ants are arboreal. In the game, they have to live underground due to map limitation.
In real life, exploding ants have 2 castes: minors and majors. In the game, the developers split the minors into workers and minor soldiers, probably to differentiate them from exploding termites.