FCOM 3.4.10 P2 defines 'S' speed as:
Minimum speed at which the slats may be retracted at takeoff.
In approach, used as a target speed when the aircraft is in CONF 1.
Represented by "S" on the PFD airspeed scale.
Equal to about 1.23 VS of clean configuration.
I'd go along with Check Airman's spin on it - not a good idea.
Cheers,
mcdhu