Feedback.The principle of operation. (Borderlands 3)

Hi.
From Mose in red branch's got talent Enhanced feedback.When Mose cricuit,the shield needs to start charging,but it doesn't always happen.Specialno test on a single mob.He damages me,the shield is damaged and waiting for sederick recharging.I shoot in the head - Cretan,but the shield begins to recharge.I understand that not so?Does anyone understand the principle of this skill?