There are 3 magic numbers in DOA4 frame data for punishing.
On guard
-
6 frames
-
8 frames
-
Anything from 13frames onwards
Everytime you block a move if the frame data says that you have 6 frames advantage. You get a guaranteed grab(F+P). On the 2nd case which is 8frames, you will get a free 7frame grab. Most characters have a 7frame grab. Eg. Hayabusa’s 7frame grab is 6F+P.
On the 3rd case which is 13frames and above, you will be able to execute your highest damage grab on your opponent.
The reason why on advantage 6, 8 and 13 is always using grabs to punish is because grabs are guaranteed compared to attacks. Usually attacks start as quickly as 10frames(P) but on the 3rd case even though Punch is guaranteed. You wouldnt really want to use a Punch. Punch does minimal amount of damage. Compared to 12 frame grabs which can do as much as 75 damage!
In DOA, punishing is one of the main damage dealer. Learn the magic numbers and start grabbing 