Hey, thanks.
the system works like this:
You have a slow health regeneration, but the enemies, especially the rifle enemies, hit you pretty often. So therefore you take them hostage and use them as a shield to regenerate your health and use their weapon/ability while you do so. They still can take damage and when they loose all their HP you drop out of them with your own health. Thats why there are 2 circles around you. To display your own HP and the controlled enemy.