Cassidy commented on this before the game and it's something Mitch has been doing forever in a variety of ways.
At the beginning of the year, he add this L cut on entries, and what Marner does really well is timing his cut into the middle to coincide with the backchecking forward meeting the gapping up defenseman. This frees up a slip pass to a linemate coming into the zone with speed (hi, Rielly) while his running mate basically drags two other players to the net (Hyman).
Cassidy after getting Marnered: