My apologies, I was referring to the aftershock variation where people pull up the rock then get rid of it by doing down,down.
By canceling the rock move you make yourself safe. For example, 21d4u4 is unsafe normally, but 21d4u4 db1dd block is safe. It also leads to pressure in that some cancels are very slightly plus (f121, f12).
People see the rock come out and often hold block a second longer, meaning you can cancel then throw, or jump over, or go for another string (e.g, f121 db1dd, f121, for some free chip and you're still plus 2).
You can be hit out of any of these, so you can only do that once they respect and try to block the rock. I like to used db1d to get them blocking low and establish that I will not cancel the db1. Then throw in a few cancels into throw or cross over jump when I need that extra damage. Blocking afterwards is fine too if you expect armour, or a fast poke of any kind.