Monster cards with the keyword Frenzy unleash their uncanny energy when the monsters are first damaged. It does not matter whether the damage comes from you or your opponent. As soon as damage is taken, the effect is triggered. For example, the Razorback Raider attacks a random enemy the first time it takes damage.
Here are a few examples of the new Frenzy cards to give you an idea of which abilities can trigger. This results in completely new combinations in the game.
You can find more information about the expansion on the official website. We are still waiting for an exact release date, but I am once again assuming a release in early / mid-April.