If I understand their diagram correctly I would guess that it is somewhat nuanced. Those shear forces are probably related to the internal angular momentum of t

If I understand their diagram correctly I would guess that it is somewhat nuanced. Those shear forces are probably related to the internal angular momentum of the proton. But in quantum mechanics you cannot precisely measure the axis of a particle's angular momentum. You can only measure the total magnitude and the component along one axis. Because of this there wouldn't be any regions you can point to that are "poles" where there is no angular momentum.