The bitcoin protocol defines the longest chain as the correct/canonical chain. At 51% you have more hashing power than the rest of the network combined, so you

The bitcoin protocol defines the longest chain as the correct/canonical chain. At 51% you have more hashing power than the rest of the network combined, so you can start mining blocks on your own and create your own chain with the knowledge that eventually your chain will be longer than the 49% chain everyone else is working on. When that happens, the 49% will abandon their chain and start working on yours.