SetEmptyAnimation(1, 0.2f); the second parameter is a mix duration. If you are not already playing anything on track 1, you aren't mixing from anything so you can pass 0.

On track 1 your code will mix from the empty animation to the stunned animation using the default mix time (from AnimationStateData). This means that instead of stunned being applied fully right away, it is applied from nothing to fully over the mix duration.

Due to alpha 0.5, the stunned animation will be applied such that you get a pose halfway between the pose from idle and stunned.