How exactly is the delta (regarding delta detection) determined?

Agent splits every file to blocks (block size depends on file size) and hashes each block individually. Then it only re-transfers changed blocks + copies blocks with the same hashes locally. Additionally, the Resilio Agent implements a rolling checksum to detect shifted blocks in case application that utilizes a file inserts / removes blocks of data, effectively shifting all the file content. If rolling checksum is successful, only inserted / removed data retransferred, not the whole file.