What is that change in tune called?
Instead of letting the song to end in the tonic (Fm) as it seems, the song chromatically ascends to the dominant (C) and recaps the hook (AbBbC Db Bb) in C as (CDE FD).
I didn't hear any tempo change but since the diminuendo phrase ends on a long note and the hook comes back vigorously through a subito glissando, it has the feeling of returning to the original tempo (tempo primo).
To create the effect, you just need to interrupt an ending with the chorus. To accentuate it, set it in a contrasting key.