Parker Fly Handbook page 19

VIBRATO
IMPORTANT! Always tune your guitar with the step-stop in the D O W N
position (i.e. with the step-stop in the H O M E position). Don't e v e r
tune it with the step-stop in the U P position. Most problems that
are encountered with this vibrato system are caused by tuning the guitar
when the bridge is not in the H O M E position. It is also v e r y important to
use a spring matched to the string size in use.
Our vibrato system offers all the options and flexibility, a guitarist could
want. The two controls known as the step-stop and the balance wheel are used
to select from the three modes of operation; balanced, bend down only, or
fixed.
One tool is needed to adjust the step-stop and the balance wheel, in order to
select one of the three modes. This tool is the 1/8" diameter, 5" long
chrome bar.