It's hard to tell from your screenshot exactly the effect you are going after. Generally there are two ways for you to give a developer control:
1) You can tell them what bones to manipulate and what values to set. For example, you can make notes of what bone to scale and what scale values to set that represent 0% and 100% of your transition. This can be OK when modifying a couple bones, but it can be difficult to get right and is easily broken if you make changes to your skeleton.
2) You can create an animation that does the transition you want. Normally the developer would just play it like normal, but you want to give the developer control over the transition. To do that you tell the developer what animation to play and what animation time to set. For this usually you make your animation 30 frames, which is 1 second at runtime. The developer sets the animation time between 0 and 1, where 0 is the start of the transition (0%) and 1 is the end (100%). This gives you a lot of flexibility because you can animate however you like and it's very easy for the developer to set the animation time and apply the animation. They don't even need to use AnimationState, they can just use Animation apply (possibly after first applying an AnimationState).
