Passthrough lets you turn the opacity of Override layers on and off. You can turn on the Passthrough option when an animation layer is in Override mode (See Setting the animation layer modes).

When Passthrough is off, the layer is always completely opaque, blocking all animation from the preceding layers with shared attributes. When Passthrough is on, you can modify the layer’s opacity to control how much animation from preceding layers is allowed to play in the result.

If you think of Override layers like a gate in the animation layer stack, the Passthrough option controls whether the gate is locked or unlocked. When Passthrough is off, the gate is shut and no animation from preceding layers can get through. When Passthrough is on, the gate can open, and its is possible for other animation to get through.

With Passthrough on, you control the opacity of the layer by animating its Weight value. The gate is unlocked, and the Weight value determines how far it will swing open.

When Passthrough is on and the Weight value is set to 100, the layer is completely opaque and its full animation plays in the result, mixed with half the animation from preceding layers. If the Weight is 0, the layer is completely transparent. None of its animation plays in the result, and the full animation from the preceding layers plays.

When Passthrough is off the layer is completely opaque, blocking all animation from preceding layers. Animating the Weight value still controls how much of the layer’s animation plays in the result.