Gheist
King of all Goblins
You're standing on a fixed spot A. You're shooting the ball into the upper right corner of the goal, B. A and B do not change. Now you shoot it there with a vertical right/left curve, a horizontal curve (lob), or add in some physically impossible movements in which the ball (or flag) gets from A to B. Trajectory changes, A and B don't. That really isn't something that's too hard to understand.[...] How does a flag with an altered trajectory land in the same place as a flag with a normal trajectory? This is where your idea doesn't make sense. If you 'spin' a football (I'm talking European here ;p) it's trajectory is changed and it does not land in the same place as it would have if you kicked it normally. [...]
Nonetheless, I don't think that any of the suggestions in here are actually needed.