Apple Final Cut Pro 5 - Manual (Page 1057)

270 Part II Project Interchange
Two-Sided Bezier handles
Keyframes that are in between other keyframes have two-sided Bezier handles. These
handles are normally locked together, so that the curve of a parameter’s keyframe graph
line has a smooth transition into and out of the keyframe, with no sudden changes.
By default, if you change one of these handles, there’s an equal change to both sides of
the handle.
The link between these two handles can be suspended, however, to create different
kinds of sudden changes in the curve. You do this by holding down the Command key
while adjusting one of the handles.
Two-sided Bezier handles for the
smoothed keyframe between a
beginning and end keyframe
Lengthening one of the handles
automatically lengthens the other.
Raising the height of the overlay curve
on one side of the keyframe lowers it
on the other, like a seesaw.
Hold down the Command key to
change the angle of a handle
independently of the other handle.