Wing tsun's "Rotation/Kicking" form with full movements/rotaions is probably developed to avoid/preempt possible bad habbits, as out/inside over/under rotations, lack of Horizontality etc.

Chum Kiu contains of nine sections describing various combinations of punching/controlling (Dar/Wu) sets of arms, Inside/Outside Superstring combinations, the Basic kicks etc.

The Chum Kiu idea is "searching for the bridge" DAR - establishing a punching line, And "Sinking the bridge" WU - avoiding the opponent to do the same by controlling his frontality by "squared" arm positions.

Chum Kiu primarily trains/tests/tricks the frontality and "no leaning back" during rotation and when kicking.