The SDS freezes the ISC plunger at step 58 so the valve won't move to adjust the idle speed as the mechanic tweaks the two throttle body air screws.
The SDS window does display some good info, like coolant and inlet air temperature and inlet air pressure, and I suppose that one could synch the two TB's by watching the numbers in the SDS window without even hooking up vacuum gauges.
However, someone who is knowledgeable about simple homebrewed instruments can make up a mercury manometer to get a direct reading in inches of mercury vacuum.
An SV650 cylinder should pull about 7 inches of mercury vacuum which equates to about 11.02 psi manifold absolute pressure and 3.5 psi vacuum at idle
The height of the mercury column will be about 7 inches if one is using a well manometer where the cylinder is pulling against ambient air pressure.
But most mechanics won't be interested in knowing what vacuum the cylinder is pulling if they can just match two columns of liquid in a u-tube manometer
The reading in a u-tube manometer will be half that of a well manometer, but the fluid used will have much less density than mercury, which has a specific gravity of 13.58 compared to water which has a specific gravity of 1.0.
Hydrocarbon fluids like oil or ATF have a specific gravity of about half that of water, so the height of a column of hydrocarbon liquid in a well manometer will be about 27 times as high as that in a U-tube manometer which will preserve its liquid as long as both ends are connected.
While the old standard for synchronizing carns with a set of Carb Stix was to keep the dancing columns of mercury within 1 centimeter of each other, one might see a difference of 27 centimeters between columns of ATF and the readings would still be in spec!
Anyway, to synchronize TB's using the Grunge Ryder Manual Idle Speed Control Device, one simply starts the engine and warms it up to 176 to 212 degrees, then hooks up the
GRMISCD to the vacuum ports and adjusts the air valve to make the engine idle at 1200 RPM.
Then opening the air screws equally on the TB's just makes the engine idle smoother and the two columns of mercury wil dance up and down very little as the RPM increases to about 1300 RPM.
Then the GRMISCD can be disconnected and the ECU-controlled ISC can be hooked up again, and what does it know?
If nobody has used the active control program of the SDS to change its set point, the engine will idle at the preset RPM