From the Super Mario Wiki
The Quick-Step Save is a Defensive Power Shot that appears in Mario Tennis: Power Tour. It is used by Mynx in the games story mode, and can be unlocked with L3 Response for Clay and Ace to use. To gain PS Response, the Reflex Rally must be used, making it available after the third ranked opposition in the Varsity Class are beaten. Once activated, the user runs on the spot, creating clouds of dust and then quickly sprint towards the ball, creating after images while doing so, and then return the ball. It is also an upgraded version of the Quick Save, reaching further balls.
"A powerful Quick Save that increases response even more."