I am constantly surprised by the difficulty that many people have in striking dou correctly. Because it works best as an oji waza they wait until their opponent makes a men attack and attempt nuki or kaeshi dou. In most cases they wait too long and hit when their training partner has entered their distance. The result is that they either strike the front of the dou or have to push both hands over to the right so that there is no power behind the strike. In either case it is a waste of time and effort as neither is classed as yuko datotsu.
The other common misconception is that dou is a diagonal cut. This probably was the case when cutting with a katana. The objective was to cut from the armpit to the hip. If however you try this with a shinai it simply slips down off the target. When combined with late timing and too close distance the result is hira-uchi, striking with the side of the shinai, which is probably the worst sin that you could commit in the long list of crimes against correct dou.
Coming back for a reality check, we need to hit the correct target with the correct part of the shinai with correct intention and “high spirit” followed by zanshin. In my view this means giving the side of the dou a good whack with the datotsu bu of the shinai ensuring that we hit with the bottom take. To do this you need to be in front of your opponent at the time of the strike and consciously punch forward with your right hand whilst turning your wrist sharply inwards. Your left hand should be more or less in line with the centre of your body. Only after you have done this should you think about breaking your grip on the shinai and moving through diagonally. It is also essential to keep your eye on your opponent and to retake kamae as soon as you are in safe distance.
One of the excuses often given for cutting dou incorrectly is that the opponent moved too quickly and did not allow us the distance to get it right. The way to avoid this is as with all successful oji waza, we should control the timing of his attack by maintaining and the breaking pressure when we want him to move. This and the flexibility of your right wrist are the keys to success.
There is an exercise developed by Chiba sensei that helps you gain this correct cutting action. You practice yoko men or dou suburi, but do so turning your hands inwards on each stroke so that the path of the shinai is horizontal or parallel with the floor. It is a way to gain the wrist flexibility that you need to make your dou attack effective.