AP is playing Izzet Spellementals and controls enough Elementals to cast Sunderflock for free from airbend, as well as enough spells in the graveyard to cast their elementals for free from airbend.

NAP is playing Airbending Combo, has executed the infinite 1/1s combo, and can get infinite Aang ETBs. Both players wish to continue indefinitely casting Sunderflock and then airbending the Sunderflock (as a spell on the stack).