How does Mixamo do the transitions in pack previews?

I'm trying to find a way to play animations sequentially with smooth transitions between them, pretty much what Mixamo does when you click on an animation pack and watch the preview. I'm using three.js and I can get transitions fine, but the problem I have is when the animations are not in place and the model moves around, my model always resets its position back to the starting point for each animation. A lot of times people use in place animations and control movement with code, but Mixamo seems to be able to do continuous transitions with animations that don't have the in place option and I'm curious how that's done. Or does Mixamo actually control all movement with code in the previews?