The engine understands exactly four easing shapes. Every Figma easing preset is resolved to one of them at export time, so the runtime stays small and deterministic.

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The four shapes

linear

Constant rate. No acceleration.

hold

A step. Keep the value until the next keyframe — no tween.

cubicBezier

A cubic-bézier curve p = [x1, y1, x2, y2]. Solved per frame with Newton–Raphson and a bisection fallback. y may exceed 0..1 for overshoot / anticipation.

spring

A damped physical spring. bounce runs 0 (no overshoot) → 1 (very bouncy).
{ "type": "linear" }
{ "type": "hold" }
{ "type": "cubicBezier", "p": [0.42, 0, 0.58, 1] }
{ "type": "spring", "bounce": 0.45 }

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Segment convention

A keyframe’s easing governs interpolation from that keyframe to the next — out-interpolation, like CSS transition-timing-function. So you set the curve on the keyframe you’re leaving: 
"keys": [
  { "t": 0.0, "v": 0, "easing": { "type": "spring", "bounce": 0.5 } },  // 0.0 → 0.8 uses the spring
  { "t": 0.8, "v": 1, "easing": { "type": "hold" } }                    // ignored (last key)
]
The last keyframe’s easing is ignored. Before the first key and after the last, the value is held.

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How Figma presets map

The converter resolves every Figma MotionEasing to one of the four shapes:
Figma presetBlinn Motion easing
LINEARlinear
HOLDhold
EASE_INcubicBezier [0.41, 0, 1, 1]
EASE_OUTcubicBezier [0, 0, 0.59, 1]
EASE_IN_AND_OUTcubicBezier [0.41, 0, 0.59, 1]
EASE_OUT_BACKcubicBezier [0.34, 1.56, 0.64, 1]
CUSTOM_CUBIC_BEZIERcubicBezier from {x1,y1,x2,y2}
CUSTOM_SPRINGspring { bounce }
GENTLE / QUICK / BOUNCY / SLOWspring { bounce: 0.2 / 0.1 / 0.6 / 0.05 } (approx)
Springs are a perceptual bounce approximation (ζ = 1 − bounce), not a re-solve of Figma’s physical mass / stiffness / damping. The motion reads the same; the exact overshoot can differ slightly. See Figma Motion coverage.