Thank you for your interest, that's a good question!
Basically, built-in Fusion movements and the animation system makes use of Movement Timer, which is Clickteam's equivalent of DeltaTime, but only used internally.
The only control you have over Movement Timer without the extension is to set the desired framerate speed in Frame Properties, and that can't be changed during runtime, only within the properties.
The extension changes that, it not only gives you the deltatime value as an expression, but also allow you to dynamically control the Movement Control framerate in real time. Meaning you can speed up or slow down time.
Note that this doesn't affect the frequency events run, which is tied to actual framerate.
With the deltatime expression (or "Movement Timer Coefficient") you can implement deltatime to your own custom movements, without any extra calculations.
As for Physics movements, I've been working closely to Clickteam to get them working in build 296, and if you really need it, I can send it to you. But it may need a bit more work since I heard from a beta tester that there is one scenario where it needs improvement (if I remember well, it's related to applying linear velocity).
Still, this is a great extension to have on your toolset, specially since I've ported it to all runtimes I could, so anyone can enjoy it!