Eject allows a user to construct a private key from their MPC shares so that they may import it into a wallet like MetaMask.

How does it work?

In order to be able to recover the private key using your MPC shares, they simply have to be "added" up. Now thats a simplification of the underlying cryptographic operation, but the takeaway here is that you can take two corresponding shares, combine them, and get the private key which is can be imported into any self-custodial wallet like Metamask.
The private key calculation operation happens on client side, and does not involve Portal's servers.
After ejecting a user private key from their key shares, Portal is no longer responsible for the security of the funds in that wallet.
We recommend to guard this operation by multiple factors of authentication as it removes the security benefits of split custody and can leave a user more susceptible to phishing attacks.

When would I want to use Eject?

Eject allows you to provide the option for your users to switch from a two share MPC architecture to a single private key while maintaining their assets. Put another way, it allows users to "export" their MPC wallet into a wallet provider like Metamask.