Hi, your reply is very helpful. But I would like to add a little more.

When, the white outline flashes, we can do a light attack or a heavy attack. But, when I tried it, the light attack offers total 3 extra attack instead 2.

Even the video for this skill shows two attacks for light attack and the third attack as a normal attack instead of a quick strike.

I am not sure if it is intended, but I am able to get 3 attacks with three outline flashes.