the speedo feed is taken from a hall sender on the cable, the sender is just behind the clocks. Its used to inform the ECU that the car is actually moving and its also used to work out mpg, road speed etc for the trip comp (as fitted).
With the engine running get out leaving it in nuetral and pushing the car the engine should rev up to between 1200 and 1500rpm.
The speedo reading doesnt go anywhere near the ignition module, the only signal that goes to and from there is the feed from the CAS, the power to the unit a spur off to the ECU, a feed to the tacho (signal only) and the HT lead down to the dizzy cap.
Anyway the reason for this feed is that when your tanking along then shove the clutch in the engine revs drop and the engine may stall due to the revs dropping to quickly for the ecu to compensate. You may have noticed it with a dodgy cable press the clutch and the revs drop to 600 rpm or something. bWhat shoul happen is the revs will drop then slow around 1500 rpm and gradually come down.
Easy way to see if its a dodgy ICV is to stop at the lights with it revign away then stick it in 1st with your foot on the brake and bring the revs down to where they should sit/slighty under push the clutch in and if the revs shoot back up then up and down again the icv needs a clean. Also check for air leaks into the inlet tract.