From the 767 AMM - cut and paste so the Boeing logo missing but .......
MAINTENANCE MANUAL
MACH TRIM SYSTEM - DESCRIPTION AND OPERATION ____________________________________________
1. General (Fig. 1) _______
A. The Mach trim mode of the stabilizer trim system provides incremental
horizontal stabilizer position in response to changes of Mach. The mode
is only engaged when airborne (after a 20 second delay) and the manual
and autotrim functions are not engaged.
B. This chapter contains Mach trim stability operation. Refer to
22-22-00/001 for a general description of the Automatic Stabilizer Trim
System. Refer to 27-41-00/001 for the component details and system
operation of the Horizontal Stabilizer Trim Control System.