You gain regeneration when you are bloodied. However, this is a Stance power, which means it lasts until the end of the encounter or you use a different Stance power.
In other words, until the end of the encounter or until you use another Stance power, any time you drop below half hit points you gain regeneration 2 + Con bonus. When you stop being bloodied, you stop regenerating. If you are later bloodied again, you start regenerating again.