The first tool in GTA 4 modding that allows you to record car paths (stunts, races or missions) and then play it all in real time. The recorded path is reproduced very accurately, even on new maps for GTA 4 or EFLC. You can change models, components, extras, drivers, ABS settings, sync speed settings, fidelity, adjust AI.
What can be done in practice? Well, for example, trying to overtake your own result in Laguna Seca.