Out of curiosity at what point was this? There is that section around the 2 minute mark where Achilles has his shield held at his back (sort of), is that the part? At this section Achilles is using his shield this way to toy with Hector and show his general distain for him.
Otherwise, I thought this was a fairly good video showing individual shield use since they use it for striking, shoves, trips, trapping weapons, as well as general defense...
If you feel otherwise, do you have any examples of what you consider how to use a shield??
I took about 3 years of medieval sword fighting (not SCA, something parallel to HEMA), and we did sword and shield a lot - I would in fact say that this is the style I'm the most familiar with (I even made my own shield haha).
The goal of the shield is to deny angles of attack to your opponent. A swordfight is, in many ways, a
fight for the centre - you want to be in such a position that hitting your opponent is easy while - due to positioning - them hitting you is hard/impossible. If you don't have the centre and you swing at your foe, they might hit your in return - and a double kill is no way to swordfight. Remember, you don't have 100 hp, a single sword blow, properly place, could be fatal.
So moving your shield out of position to attack is foolish, and using it to attack even more so. In fact, part of the art of mastering the shield is learning how to attack
and keep your shield in position. And these shields metal are
heavy - ok sure, maybe someone like freaking Achilles can use it like that, but irl these shield bash with the edge are slow and
expose you terribly.
So for example, look at the dodge at 1:06 , followed by Achilles trying a shield bash with the edge at 1:07. Had Hector's footwork not been so inept, Achilles is WIDE OPEN. His spear is on the other side, his shield is up high as is his arm. Hector could have stabbed him right in the side of the torso, maybe even in the unarmored armpit! But Hector's footwork and block are inept, leaving him open too - but Achilles just... runs on.
Then at 1:14 there is this running leap with a spear trust, very dramatic... except that the spear trust is
right at the middle of the shield. Why hit at the middle of your opponent's defence? And it wasn't some kind of opening move, a combo, Hector just staggered back as Achilles watch. Had Hector turned his shield slightly, the trust would have been deflected and he would have had absorbed a lot less of the impact of the blow - AND HE COULD HAVE STABBED ACHILLES MID AIR.
I'll note at 1:18 that there is an interesting block by Hector. Low attacks to the legs are
not normally parried by the shield like that (your legs can move fast, it opens you up) but for a moment, he pins Achille's spear to the ground, and, for... no reason I can discern, Achilles's shield is (once again), out of alignment and is basically wide open. HECTOR COULD HAVE STABBED HIM THERE TOO but once again, Hector seems incapable of doing much but stagger around.
These fights are... done for entertainment, for drama. And that's ok! we want to be entertained right? But, they aren't realistic, and should be used as "evidence" of how these weapons were used with great caution.