Donna holds her sister Alyssa unreasonably tightly. They're positioned wrong for her to be able to feel her sister tell her to calm down but she can infer it. They know each other well.

They confirm they're ready. They go.

The thing about choices and actions is that there's always a delay between them. Someone with truly instantaneous reactions could afford to make the first jump intending to decide what to do next in the other world.

There's time for Alyssa to catch a glimpse of the world before Donna brings them home.