....the purpose is to reduce hammer strike but still achieve high power.
I have seen more than once where it was implied the existing valve was balanced. I do agree that there is less force on the closing than the typical valve set up with the valve rod being exposed to atmosphere but it is not balanced the way it performs. I have an idea on how to make it balanced also but the back of the action would need to be opened up and again lots of time on the lathe to get extra orings to seal. Not that the Impact needs any more orings . There are already too many IMO.