https://en.wikipedia.org/wiki/Z-drag

A picture (diagram) is worth a thousand words. Ascenders can be substituted for the prusiks, although a properly tied prusik with good rope is usually stronger. This is a standard rig in SAR, typically employed is moving a loaded stretcher up a slope. There is a theoretical 3:1 advantage, although friction does take its toll, especially if you have no pulleys and have to substitute carabiners.