All the proper and allowed Do strikes
I always assumed that Do had the intention of cutting someone in half (kind of), so any proper Do must be executed diagonally (same applies to both ordinary and Hiki techniques). Am I mistaken?
You are right about the idea of do strikes even though it would be probably pretty hard to cut a person’s body in half with one strike. However, we must strike do very hard.
It is said that we need 20-30% of our power to cut a kote, 60~80% to cut a men, 100% to cut a do. That is why we need a good aim too. You do not want to miss a do strike with your full power on your training partner.
Do strikes (including hiki do) should be diagonal. That is what we learn at the very beginning so it is the basic do strike.
It is because the diagonal cuts are more logical. You bring your shinai up above your head and strike the right do of your opponent. To strike the right do, the blade should be diagonal.
Hope this helps.