How does the Conditional field works ?

[RamaHerbin]

I reviewed the block_video component and I don't understand how the type conditional field works. Usually when I want to have a field that changes dynamically according to an input value of a radio button, I add a condition to the field that depends on the value, and if the value corresponds I show and hide fields accordingly. But in the case of the type file in block_video I don't see any condition.
So I'm wondering how the dynamic field is working ?

I hope that what I wrote makes sense.