For those who were wondering:
She has a special pull animation with the screen turning off and her model showing up with her back turned to you which appears on the last screen before your gacha pull results (when you see the colored orbs), much like Madokami.
The gauge is a one time thing per banner and it's an extra pull after the 100.
Summer Mami works best with her A1 as it'd take a bit to get her magic to 5 and maintain it there in harder content.