I spent 2 hours one night in the practice mode trying to discern the difference between the 3 levels of assistance. This is what I found:
Manual: Full range of motion of the stick for placement of a shot. Extremely difficult when using the Tele camera..LOL!
Semi: Half the range of motion - meaning your shot will always be directed toward the right end of the pitch but the range of "forgiveness" in your shot is greater.
Assisted: Shots will always go toward the goal. You still seem to have control over placement - i.e., if I hold the stick left the shot will go to the left. If I hold it upper-left, the shot will go upper left. You can still miss badly on assisted.
I really think it all has to do with how much room you have to move the stick. I can't place a shot on manual settings to save my life. I could in 09, but I also used the Dynamic camera which put me closer to the pitch. When zoomed out, you have a lot less space to work with on the stick making manual controls a bit more difficult. I've noticed this a lot in passing as well.
YMMV but those were my observations. I've been playing with assisted shooting now. Still can't score to save my life, but have placed a few shots that I know would have sailed to a different continent if I still had shooting on manual.