If you start pushing on the top of the blade with your top hand, we know that every stroke you will be lifting water and when you lift water the surf ski drops down and causes drag. The top hand needs to stay relaxed and above eye level, so that the paddle head that is under the water stays vertical. The paddle head under the water pulls vertically against the water and you rotate forward past the blade.
As you rotate through the stroke the top hand will stay above eye level and can come through to the opposite rail of the surf ski. Knowing the mark X you are aiming for each stroke gives you something to aim for and hit every single stroke.
We do not want to push the top hand forward as the paddle head that is under the water starts to lift water causing the surf ski to drop and lose speed. We want to keep the top hand above eye level and keep the paddle head that is under the water vertical, so it is pulling against the water and the paddler can rotate and drive the surf ski forward.