I think they had to Retraction Helix infinite times a 1CMC noncreature spell with a mana dork tapping for mana and being untapped by the ascendancy. But they were just casting infinite times without affecting the board. They needed to have either Altar of the Brood or a prowess card to attack for infinite
Edit: @Dunadain I actually got to reach him! here how it is
Sylvan Caryatid + Jeskai Ascendancy+ Altar of the Brood in play
you cast Retraction Helix now you can tap your Sylvan Caryatid to bounce a nonland. You play Astral Cornucopia for 0 and your opponent mills 1 thanks to Altar of the Brood , you tap your Sylvan Caryatid to bounce the Astral Cornucopia back to hand. and play again, when you play Jeskai Ascendancy triggers and untaps the Sylvan Caryatid , and there you have a infinite loop I think
Double Edit: The modern combo isn't infinite? how it is?