I picked this up today for $0.99. The GBA snaps into the center of the wheel. However, when you steer, it triggers these pushers to push the D-Pad left or right so you turn as you steer. Same thing with the A and B buttons.
I have Dance Dance Revolution for Gameboy Color with a snap on dance pad that fits over the Gameboy Color and basically does the same thing. When you press up on the dance pad, a trigger pushes up on the D-Pad.
The reason I am mentioning that is because after playing Dance Dance Revolution a lot on my Gameboy Color, it really wore down both the D-Pad and A and B buttons because of the stress the triggers from the dance pad put on the D-Pad and buttons.
I suspect the same thing will happen with this GBA steering wheel where if you use it a lot, it will wear down both the D-Pad and A and B buttons from the triggers putting stress on the D-Pad and buttons.
I ended up using the steering wheel for 1 lap of a race and then decided I don't want to ruin my GBA so I stopped using it. It did seem to work ok but it wasn't worth it to me to use it, even if it did only cost $0.99