SHA takes in chunks of data at a time. If you know the first X chunks of data will be the same (block template pieces like tx's) then you input that stuff first, now you have a 'midstate' or checknpoint to start from. You don't need to rehash those initial chunks. If you're doing SHA256 for secure comms, this is retarded and dangerous. If you just want PoW, it helps save ~20% work