Plasma reduces what needs to be trusted by moving most activity off the main chain while preserving the ability to exit back to it with cryptographic guarantees. The design avoids complexity bloat by keeping the security model simple - users can always withdraw their assets unilaterally if something goes wrong, regardless of what happens on the Plasma chain.
The core mechanism is exit games. Users commit assets to a Plasma chain that processes transactions quickly and cheaply off the main chain. If the Plasma operator misbehaves - censoring transactions, including invalid state transitions, or going offline - users can submit an exit proof to the main chain showing they own specific assets. There's a challenge period where anyone can dispute invalid exits, but valid exits can't be blocked. This means users don't need to trust the Plasma operator to be honest, only that Ethereum remains available for exits.
What this avoids is the complexity of consensus mechanisms, fraud proofs for arbitrary computation, or validity proofs for every transaction. Plasma chains can use simple authority-based ordering because the security doesn't depend on that authority being trustworthy. The operator can't steal funds even with complete control over transaction ordering because users hold exit proofs that the main chain will honor. Trust requirements collapse to "can you get a transaction on Ethereum" rather than "is this entire secondary system operating correctly."
The tradeoff is data availability. Users need to watch the Plasma chain to construct exit proofs, which creates liveness requirements that other L2 approaches avoid. But for applications where users are expected to be online regularly anyway - payments, exchanges, gaming - this is acceptable. You gain massive throughput and reduced fees without introducing cryptographic complexity or novel trust assumptions.
Plasma represents efficiency through constraint rather than generalization. It doesn't try to replicate full Ethereum functionality off-chain, just the specific use case of asset transfers with main chain security. @Plasma #Plasma $XPL