by tension you mean the red sem icircle that fills up? its the tension/SP bar. basically like i nthe demo you go into a darling slave mode in which your weapon starts to steal the life force of anything it comes into contact with.
your Hp goes down gradually since you are holding it, and its refills your HP if you attack a monster. the cost is fair considering you get a stagger free boost, unlimited stamina, and an increase in attack and critical ratio