For this kind of thing I find the SampleCSEventWatcher developer sample really handy. If you clone that repo and build it and then run the SampleCsEventWatcher command, it’ll list all of the events to the command line as you do an action. For example, for dragging a curve with the gumball, I see:
Basically, OnBeforeTransformObjects fires with the transform matrix, and then the curve geometry is replaced with the transformed version in ReplaceRhinoObject.