So the sequence of events is as follows.
You declare target.
You roll to hit
If the unit is hit I Declare Grot Shields
You roll to wound initial target
For each successful wound, I roll a 2+
Each successful 2+ does a MW to a Grot (ie, remove a Grot, do not pass go, no 200 teef)
On a failure of a 2+ I roll save if any