In short, I look at the hands and what they do. The plane they travel from down the line, and lag - sequence - (or lack of it) from face on.
And balance, the key indicator of a smooth swinging motion, regardless of hitting or swinging is being used. Best to see in the footwork and finish position.
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"Support the On Plane Swinging Force in Balance"
"we have no friends, we have no enemies, we have only teachers"
Simplicity buffs, see 5-0, 1-L, 2-0 A and B 10-2-B, 4-D, 6B-1D, 6-B-3-0-1, 6-C-1, 6-E-2