Cast any other instant and/or sorcery spell, or wait for an opponent to cast one.
Holding priority, cast Expansion by paying {U/R}{U/R}, copying the next instant or sorcery spell you cast with mana value four or less.
Cast Twincast by paying {U}{U}, targeting the instant or sorcery spell cast in step 1.
The delayed trigger from Expansion triggers, copying Twincast, with the copy targeting the original Twincast.
Resolve the copy of Twincast, copying Twincast, with the copy targeting the original Twincast.
Repeat step 5.

Infinite magecraft triggers

Expansion // Explosion

Twincast

Cast any other instant and/or sorcery spell, or wait for an opponent to cast one.
Holding priority, cast Expansion by paying {U/R}{U/R}, copying the next instant or sorcery spell you cast with mana value four or less.
Cast Fork by paying {R}{R}, targeting the instant or sorcery spell cast in step 1.
The delayed trigger from Expansion triggers, copying Fork, with the copy targeting the original Fork.
Resolve the copy of Fork, copying Fork, with the copy targeting the original Fork.
Repeat step 5.

Fork

Cast an instant or sorcery spell, or wait for an opponent to cast one.
Holding priority, cast Expansion by paying {U/R}{U/R}, targeting the spell.
Activate Rootha by paying {2} and returning it to your hand, copying Expansion, with the copy targeting the original Expansion.
Resolve the copy of Expansion, copying Expansion, targeting the original Expansion.
Repeat step 4.

Combos with "Expansion // Explosion"