The principle of geth checking whether a block has the highest total difficulty

Unsure about the exact mechanics of how that works, but the Rollback() function can be found in blockchain.go. (There's also a version for the "light" client, here.)

// Rollback is designed to remove a chain of links from the database that aren't
// certain enough to be valid.
func (bc *BlockChain) Rollback(chain []common.Hash) {
...