Expansion // Explosion + Twincast

Prerequisites

You have a way to cast an additional instant or sorcery, or an opponent is casting one.

Steps

1. Cast any other instant and/or sorcery spell, or wait for an opponent to cast one.
2. 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.
3. Cast Twincast by paying {U}{U}, targeting the instant or sorcery spell cast in step 1.
4. The delayed trigger from Expansion triggers, copying Twincast, with the copy targeting the original Twincast.
5. Resolve the copy of Twincast, copying Twincast, with the copy targeting the original Twincast.
6. Repeat step 5.