The technique of the nollie heelflip mixes that of the regular nollie and that of the heelflip as you might expect. As with other nollie tricks, it can help to visualise this trick as a switchstance fakie heelflip.
In order to do this trick with power, you need to nollie as high as you can first and then kick out with your heel to flip the board after you’ve left the ground. Practice makes perfect as always.