The icon moves by itself, going left and right, left and right. If you panic and try to correct it under the assumption that it'll continue moving left, you'll just overcorrect and send it careening in the opposite direction instead, and once you get behind, you'll likely find it difficult to come back. The trick isn't to keep the fist icon in the center, but to keep it moving left and right within the boundaries of the area you have to follow. You do realize that you control that fist icon and not the bar? That bar is just the area you have to keep the icon within. Just making sure.

Remember that your icon is sluggish and has momentum rather than directly following your mouse's movement. With that in mind, subtle movements work best. Gently moving your mouse at first and when it switches direction is necessary, but it shouldn't change direction often (usually just once, if you stay within the boundaries the whole time). Beyond that, it doesn't require that much actual input. Once you get the icon moving at the same speed as the bar, it should be moving left and right well within the acceptable area.

If you use large motions, you'll most likely lose. It's all about subtlety and small corrections.