This is part of an advanced setup. It is not required for most riders.
One will need a couple feet of fishing line and a fishing weight as pictured below.
While the pedal is in the 3 o'clock position the rider should have the fishing line hang from the bottom of his/her patella tendon ( as pictured below ).
The weight should contact the rider on the top of the foot, directly above the ball of their foot as pictured
Slide the saddle back, if the weight is in front of the ball of the riders foot, or slide it back if the weight is behind the ball of the riders foot.