The topic is a bit tricky so let’s do a quick recap how it works and what can be done to get an instant animation transition.
Under the hood, Sketchfab engine supports only linear interpolation, so if we want constant/step keyframes (sharp animation), we need to trick.
During the processing, all animations are baked with a constant and defined framerate. The reason is that file formats and softwares have dozens of features/modifiers that can alter the animation of a given object, in addition to it’s own keyframes. Baking everything into a new animation track allows to take into account as much features has possible. The problem is that we will always have the same distance between two keyframes, that will lead to smooth interpolation.
Workaround for step/constant keyframe / sharp animation
For both glTF and FBX file format, when a constant/stepped keyframe is detected, we bake two keyframes with a very small interval between them in order to recreate the sharp animation transition.
In order to get this working, the keyframes inside the file must be explicitely declared as constant (FBX) or STEP (glTF).
This workaround works well as soon as the data is inside the file.
In some cases, the software doesn’t export this data on the keyframes into FBX files (I need to check but I think it’s not exported from Blender).
The case of Blender
(@shaderbytes you probably know Blender much better than me so don’t hesitate to comment if I missed something)
The tricky part with Blender is that it plays animation frame by frame, so whatever the type of the keyframe, you will always have a sharp animation between two successive frames. On Sketchfab, if you use the exact same data, you will have an animation between the two successive keyframes, defined by the interpolation between them.
If you want to reproduce the same behaviour on Sketchfab, just clik on the time in the timeline to switch play mode to ‘frame by frame’. You should have sharp transitions now.
At the moment, there is no workaround to support this for