>>11226649he's basically right. if you are doing pullups and the rest of your body is oscillating in any way then some of your arm strength will need to go into stabilizing those oscillations. otherwise you keep swinging more and more and your muscles need to keep adjusting and flexing to account for that, which requires work.
being completely tense helps to stop oscillations but you also need to make sure your entire tense body isn't rocking side to side too. a rigid thing suspended at two points dissipates energy too -- imagine you had a weight hanging on a double hook where the hooks form a y-shape. if you displace the weight along the line of the bar supporting the two hooks, then one hook needs to raise off the bar supporting it, then once you release the weight the hook that's raised smacks down on the bar and makes a loud clack which radiates off sound energy, and if it continues oscillating you get multiple clacks that radiate away energy.
so in other words when doing pullups, the most efficient thing is to move up and down perfectly rigidly with no forward/backward or left-right oscillations because that sucks away energy in the form of flexing or etc
