First of all, be aware that GPS modes can be thought of as variations on Attitude mode.
CourseFeedForward
VelocityFeedForward
I confess I haven't played with these and don't have the code open. They are supposed to make the FC calculate a little ahead and use the prediction of course position or velocity instead of the actual course position or velocity. Without checking, I think that means for instance that one second before getting to a waypoint, it switches to the next waypoint.
Yawcontrol is whether you control the yaw in GPS modes or whether the FC does
- point the nose always toward next waypoint
- point the nose always in direction of motion
- allow user to control the direction the nose points
It just flies sideways or backwards as necessary as the user rotates the yaw. Beware that some of these yaw modes do strange things in some GPS flight modes. Example: if you set yaw to be direction of motion and you are motionless, the yaw jumps around wildly. That can cause a crash in a poorly balanced poorly tuned multicopter.
FlyawayEmergencyFallback is used when the mag is determined to currently be unreliable (so it doesn't know the compass heading). It is supposed to set the Attitude mode pitch to 15? degrees forward to guarantee forward motion, and use the direction of motion to determine the compass heading. So if your mag is bad, it can still be flown back to you.