The remote hi/low speed adjuster has a low speed needle and a high speed blow off valve and as you adjust the screw tighter it increases the low speed compression and also increases the preload force on the blow off valve thus giving the shock more high speed compression. If the shock experiences a large high speed hit it will still blow off to prevent harshness and prevent the shock from hydraulic lock (keeps the shock from blowing up).
You can run the adjuster at full hard to simulate lockout and this improves pedaling while still absorbing larger hits.