A solid, minimal-assumption description of value handshakes. This SSC post contains the best description of which I'm aware, which I think is slightly sad:
Values handshakes are a proposed form of trade between superintelligences. Suppose that humans make an AI which wants to convert the universe into paperclips. And suppose that aliens in the Andromeda Galaxy make an AI which wants to convert the universe into thumbtacks.
When they meet in the middle, they might be tempted to fight for the fate of the galaxy. But this has many disadvantages. First, there’s the usual risk of losing and being wiped out completely. Second, there’s the usual deadweight loss of war, devoting resources to military buildup instead of paperclip production or whatever. Third, there’s the risk of a Pyrrhic victory that leaves you weakened and easy prey for some third party. Fourth, nobody knows what kind of scorched-earth strategy a losing superintelligence might be able to use to thwart its conqueror, but it could potentially be really bad – eg initiating vacuum collapse and destroying the universe. Also, since both parties would have superintelligent prediction abilities, they might both know who would win the war and how before actually fighting. This would make the fighting redundant and kind of stupid.
Although they would have the usual peace treaty options, like giving half the universe to each of them, superintelligences that trusted each other would have an additional, more attractive option. They could merge into a superintelligence that shared the values of both parent intelligences in proportion to their strength (or chance of military victory, or whatever). So if there’s a 60% chance our AI would win, and a 40% chance their AI would win, and both AIs know and agree on these odds, they might both rewrite their own programming with that of a previously-agreed-upon child superintelligence trying to convert the universe to paperclips and thumbtacks in a 60-40 mix.
This has a lot of advantages over the half-the-universe-each treaty proposal. For one thing, if some resources were better for making paperclips, and others for making thumbtacks, both AIs could use all their resources maximally efficiently without having to trade. And if they were ever threatened by a third party, they would be able to present a completely unified front.