DA does 6% on the initial hitbox, and 4% i think on the later hitbox.
It comes out really fast, even from a standstill if you use the Control stick and the Cstick downwards.
I'm not sure on the frame data, but DA should be able to punish nearly any move that hits Sonic's shield, plus the low knockback from it makes followups easy. The late hit combos into Utilt extremely well, even at high percent, although it is pretty hard to land. You can also set up a spring under the opponent if you hit them with the weak hit, or just run back under them if you pop them up with the late hit.
So basically, use it to punish any laggy attack, but be careful at low percent since some characters can hit you with a bair before you can shield at certain percents.