I don't think this HUD is anywhere near final, it's missing the weapon selection menu and other things like the blue Warp meter.
Anyway, the constant attacking here works just like in Type-0, it's like Type-0 where you can press the button once to do the commands, so you press attack once and it will do attack once, or hold attack down to prolong the attack period, pressing attack once = one sword swing is also how KH works and it's still visible here, it's also doing it like in Type-0 though so you can also hold attack down to continually attack.
Some commands in Type-0 have a charge on them so holding them down will only charge them instead of continually attack like regular weapon attacks, this is mostly for magic though, the Fire spell Noctis used seems to work that way too, he did a small charge on the Goblin, and then later a longer charge on the Behemoth near the end.
The "defend" will not always evade damage, there was multiple instances where Gladiolus and again later Ignis is on "defend" yet they still received damage as shown by the red on their name, so you can't get by just by holding it, this "defend" would also be similar to in Type-0 where you can hold down dodge and a direction on the stick, but even in doing this you can still take damage in Type-0, and you can also time the dodges in Type-0 to evade attacks like you would say in KH, how it works here would be the same context, so you still can time dodges like you can in Type-0.
It's very clearly a mix of the Type-0 and KH like system, which I thought everyone already knew it was going to be.
Pics of Gladiolus and Ignis taking damage while in "Defend"
Also you can hold down A/X/O to continually input commands in most if not all previous mainline FFs.
