Each point of vitality provides you with one Healing Reserve, which is an expendable resource that represents your ability to push through pain and reinvigorate yourself to keep fighting. Various actions allow you to spend a healing reserve; when you do, you regain hit points equal to your heal value, which is equal to half your bloodied value, rounded up. When you take Vitality damage, you also automatically lose any reserves in excess of your new Vitality, which you cannot recover until the Vitality damage heals. If you have already spent more reserves than your current vitality, you do not lose a reserve when you lose vitality - the reserve that you would lose was already spent.
Whenever you take a short rest, you may choose to spend any number of healing reserves, regaining hit points equal to your heal value for each reserve that you spend. If you have per-encounter feats that allow you to regain additional hit points when you spend a reserve, you may use each of those feats once during the Short Rest.
Whenever you take an extended rest, you regain a number of hit points equal to your heal value times your remaining reserves, then regain 1 point of lost Vitality if you have taken any Vitality damage. You then regain all reserves up to your current Vitality.