Unlike the normal "match" control scheme, Freekicks are "finesse" by default. Holding LB (player switching button) delivers a power freekick.
To have the ball dip behind the wall, you need to get lots of topspin, and just the right amount of power to that the ball clears the wall but dips under the bar.
THe Freekick system is kind of weak since they only use the power gauge as a control for both power and height. While more power usually gives more height, I feel this system robs the player of alot of detailed control over the shape of the freekickers foot and such.
Basically, in real life, just by changing the shape of the foot, you can get a variety of different heights for shots with identical power. In FIFA you basically only have 2 height possibilities for any amount of power (1 higher arch for finesse freekicks, and 1 lower arch for power freekicks.
PES, by using both the d-pad (up, down or center) and additional buttons (Y, X) for each d-pad possibility, allows for 9 different trajectories at all times.
FIFA really needs a new system, so that freekicking can be made both more complex, and easier to use (in the sense that you'd have more control).