Animation Interpolation with Quaternions

We want to try another algorithm (that we are going to implement on our own) for Animation Interpolation using the Unreal Engine (Our version is 4.7.6).
We have already done our tracing into the code, but for different animation testing we see that different code is called, so we are a bit baffled.

Is it possible to tell us which function is used in the Engine to calculate that?

Thanks in Advance.