Parker Fly Handbook page 22

• The Bridge Pivots on Ball Bearings.
• Bridge Needs to be Balanced at all Times.
THERE ARE 3 WAYS TO DO THIS:
The following things can affect your bridge being level in the H O M E
location; breaking a string in balanced mode, alternate tuning, changing
string gauge, moving the step-stop, or truss rod or action adjustments.
There is only one position for the bridge to be in no matter which of the three
modes you choose. It must be "level" with the strings in the HO M E location
whether the step-stop is engaged or not. The correct scale length location
and intonation depend on this.
The guitar strings pull the back of the bridge up toward the headstock.
The tremolo spring pushes against the bridge.