I wouldn't be surprised if it's similar, that is, single attack is a swipe, with follow ups and all that. Whirlwind already works like that, if you keep holding the button you'll keep whirlwinding, no reason this ability can't be the same, consuming stamina per attack. It has it's own icon though (and you see it getting used with a very quick CD in the MP video). It's the Rage icon, most definitely not the Devour or Aura of Pain icons. Why I think the thing you see Iron Bull doing - where he very specifically 'rips away' from the enemy in the attack, something you don't see in any of the 3 attack chains regardless of when they end - is most definitely devour.
Ahh I really wanna know the full Reaver skill tree.. well I want to know all of them lol. But Reaver is gonna be my main character. I like using it to create a high risk/reward 2H tank. Be a little more interesting considering unlike the other specs they can actually heal there base health. Which, apparently Devour gets bonus something based off if targets are effected by Aura of Pain. Least in MP anyway, so you have to harm your self, to do toggle on a DMG aura so you can get a more powerful of devour which you probably need because you've been hurting your self lol. The cycle of pain and healing continues. 
-edit-
@We Stand Midnight Clad : No, no they haven't. The ^ at the top (singular and small) are for stuff like Blizzard or Stealth. It's a toggle, as close to sustained as we have from past games. You click it, it toggles on, and stays on till your out of resource, it's duration is up (stealth), or you click it again to toggle it off.
The Larger V on top/bottom of an icon is used for something you have to manually hold the button down to keep going. Shield wall and Counter are good examples of this, you can hold it down and it'll suspend the CD of the skill till you stop (in the case of Whirlwind). Something with a little sparkly *-ish at the top means its a detonator.
Recent Examples : http://imgur.com/a/DzqMh - and there in the recent twitch streams : https://www.youtube....gVd8zKKM2NnPDRg
That said Spirit Blade has none of those, not even the *, but it lists it as a detonator sooo... I feel like they kinda forgot that icon's indictaors all together lol. Does look (and sound) like hes pressing the button for each use of the spirit blade though in that video, could be a toggle though - but again, you don't see the blade unless he attacks so I have a hard time believing its a toggle.