So if you are stunned you can't use attack maneuvers, you can parry at half and you get -50 on all other maneuvers (but your DB is untouched).
If you are critically wounded for a -50 penalty to maneuvers, you get -50 on St, Ag, Qu maneuvers only and you lose your Qu x 2 to DB (unsless your Qu bonus is higher than 25).