Challenge 2: A Sharp Punch Looking Soft and Floaty With Default Bezier Interpolation — Solution Walkthrough

The answer:

Per this chapter's own warning box, this is exactly the mistake it
describes — leaving every keyframe at Blender's own default Bezier
interpolation regardless of what kind of motion is actually being
animated. Bezier's own smooth, automatic easing softens the sharp,
sudden quality a punch actually needs, making it feel gradual and
gentle instead of quick and forceful.

Why Bezier specifically works against a sharp motion:

Bezier eases in and out of each keyframe smoothly, gradually
accelerating and decelerating rather than moving abruptly. A punch's
own defining character is exactly the opposite — a fast, sudden
acceleration with little or no gradual buildup — which default Bezier
easing directly undercuts.

What would fix it:

Per this chapter's own material, switching to Linear or even Constant
interpolation for the punch's own key moments would produce the
sharper, snappier motion actually needed, rather than leaving the
motion's own timing at a default setting better suited to smoother,
gentler movement.

WHY THIS WORKS AS AN ANSWER
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This exercise checks that a soft, floaty result on a sharp motion is
correctly traced back to leaving default Bezier interpolation in place
without matching it to the actual kind of movement being animated.