Design: https://github.com/omgnetwork/plasma-contracts/blob/master/plasma_framework/docs/quasar/quasar-fast-withdrawals.md
Implementation: https://github.com/omgnetwork/plasma-contracts/tree/master/plasma_framework/contracts/quasar
This is an attempt to reduce the time needed to move funds from Layer 2 to Layer 1 to <= 24 hrs. It is a trustless way to exchange funds using a Layer 1 smart contract, which would enable almost anyone (i.e. not only limited to token-issuers or partners) to provide the service for incentives. This approach also pushes for bringing the liquidity providers to the exiters, hence eliminating the requirement of a token market.
Exchanging/Swapping assets is done with the help of transaction inclusion proofs
To put it briefly, the UX for Alice, who has PETH (ETH on Plasma Layer 2) and wants Layer 1 ETH is: