With the pedal in the open position, stick a small hook or screwdriver in to grab the spring. (Be aware that the pedal does not detach from the base, so once this modification is done, putting the spring back in can be tedious).
Pull the spring out and remove from the pedal.
Place the front end of the pedal so it touches the base (top down position).
Look on the back side of the HB01 (the heel down side) to locate the rocker screw.
Using an allen wrench, tighten the rocker screw to your preference of resistance (if you do not do this step, the pedal will fall towards the toe down position whenever your foot is not on the pedal).