To recreate this you need to do a normal punch and then immediately when that animation is playing spam for a heavy attack. Watch the stamina bar, it will use the right amount for the first normal punch but for the second heavy attack it will not use any stamina.
https://www.twitch.tv/dellor/clip/TastyRacyBoarDatBoi-JeZEoFd7sjssFsA3