I just recently unlocked it on my new slayer. Though he is only level 20 I ran some tests in the gauntlet.
I compared slash, whirlwind and strike. I used activity 10 and 7 with level 1 bloodlust. The gladiator uses a sword in main hand and a dagger off hand.

It looks like strike has a bit fewer attacks then slash at 10 and 7, while whirlwind has the same number of attacks at 10 and has more at 7 then slash.

Strike looks to do less damage but more criticals (as expected).

Whirlwind does usualy 3 attacks on first round and three the second while simple slash does 2 on first round and 2 or 3 half the time on second round (activity 7).
Also Whirlwind seems to do more damage, but I'm not sure.

On activity 10 both whirlwind and slash does 3 attacks on first round (slash sometimes 2 and whirlwind sometimes 4), but whirlwind did 4 attacks on second round more often.
Strike usualy does 2-3 attacks.

So I can say it increases the number of attack a bit, probably it doesn't compromise defence like berserk does (still it is an offensive style so defence skill trigger less obviously).

I would say it is like using Slash/lunge/bash, but uses more stamina and incraises attack rate a bit. Probably has the same defence modifier and defence skill trigger modifier.