Fantastic Elasticity

Modality married to rebound is the whole trick: you make the choice twice, once now and once on your next upkeep, and the two casts do not have to agree. The first mode is a straightforward bounce; the second reclaims an instant or sorcery from your graveyard, and pointing the same card at different jobs on consecutive turns is where the value lives. Cast it as tempo to unseat a threat, let rebound fire, then flip to the graveyard mode to buy back a burned spell (potentially a copy of this very card if you cast it from hand again, since exile-to-recast lets the sequence chain under the right conditions). Rebound doubles a single card into two effects by design, so the interesting constraint is not that it costs you value but that it demands a plan: the second cast is a promise you make on the first turn, before you know which mode the board will want next upkeep. Note that rebound keys strictly on a hand cast: any copy of this spell on the stack duplicates its chosen mode fine, but a copy is never cast, so it never sets up the delayed second casting. Only the original ever comes back around. That structure rewards taking the bounce now when raising your own graveyard spell right now would be the weaker line, since the recursion half is already guaranteed to arrive next upkeep.