There are two modes to figure out what you're looking at. By default, we select "Highlight", a mode that highlights the removed parts in red and the added parts in green, leaving what is unchanged as a wireframe.
Also available is the revision slider, which lets you transition between the current version of the file, and the previous one with a small slider at the top of the viewer.
How does this work? We take both versions of the model, and using binary space partitioning, we compute the added, removed, and unchanged parts. This is done using csgtool, a C library paired with a Ruby gem via FFI. These pieces are cached and displayed by the 3D viewer we already have, though we color them differently and play with their transparency to help illustrate the changes. If you have any further questions, check out the help article.