Fast-Slow speed indicator in B732

[md-100]

Condition:

A/c : B732
Autopilot : Sperry 177

Situation:
A/T : Disengaged
A/P : Disengaged
F/D : On
APP Mode : Capture (LOC & GS)
MCP Speed : Selected Target Speed

Question:
How does the FAST-SLOW SPEED INDICATOR work?

Thanx

I forgot..... Does anyone know where I can find a Sperry 177 manual or operation in the net?

thanx again