Yes a bit confusing when you first use HDX or Varicam..
The half shutter means its giving you a 180 degree shutter angle for what ever you have set the camera for… ie a half.. so at 25p half shutter is 1/50th.. just saves you having to work it out yourself.As far as i know anyway !!
And in theory replicating a film camera.. sometimes I,ll switch it off if I need a bit more stop.. as long as not alot of action in the scene its fine..