When a VM backup starts, QEMU installs a "copy-before-write" filter in its block layer. This filter ensures that before new guest writes occur, any old data still needed for the backup is first sent to the backup target. The guest write operation is paused until this transfer completes, meaning that guest IO for sectors not yet backed up will be limited by the backup target's speed.
With Backup Fleecing, however, this old data is cached in a fleecing image instead of being sent immediately to the backup target. This approach can improve guest IO performance and even prevent potential hangs, though it does require additional storage space.