Nioh: How Attack Increase (Timely Guard) works 

You don't need a perfect guard skill weapon for it to work (works with any weapon)
Timing is pretty precise but, the if you mess up, just hold block to continue blocking the attack and try again
Works on blockable projectiles too
Stacks with the other attack buffs (Bonus when Surrounded, Bonus from Amrita Absorption, Summer Twilight)
Works in any stance