My interested-lay-person understanding: Previously it would read a chunk of file, then decode it, then apply filters, then run multi-threaded encoding, then write a chunk of file.

Now it can read the next chunk of file while decoding a chunk while applying filters while running multi-threaded encoding while writing the previous chunk.

I expect this will be a big help for complicated setups (applying multiple filters, rendering to multiple outputs with different codecs), but probably not much change for a simple 1-input 1-output transcode.