hitting sweetspot
I find that my ability to hit the sweetspot is equal to the degree to which I can keep my head still before and during the swing. Running with a jarring, up-and-down action causes my head to bounce up and down, causing dizziness. Jarring my head during the swing causes the ball (and the whole environment) to appear to jerk away suddenly to a different position.
But if I can keep my head still, then it is easy to see the ball clearly, and to predict its path to my racquet.
Staying balanced also helps, because the feeling of falling can cause me to make a sudden involuntary adjustment that upsets my intended stroke.