Note:
The following is based on my personal observations and does not include any Superchips data or opinions.
Introduction:
The main feature of the Superchips modified ECU is the addition of a mini PCB in the idle circuitry.
This is shown in the pics below. I could not find evidence of any other changes, but this does not mean there aren't any.
close up pic of a mini PCB
and another one
Superchips PCB data:
Below is a circuit diagram with standard aux PCB circuit & components and manufacturer recommended settings.
Aux PCB (A, 
is fitted on 2 legs in place of R345 & R346 which are in parallel. The yellow lead running from R1 is soldered to the track which runs from ECU connector pin 2 (throttle closed switch).
Standard ecu R345/6 measurement is ~1200 ohms (in circuit).
R345 = 2740, R346 = 2200.
the circuit diagram
R1 - Reads 1k but measured at 10k
R2 - 4k1
R3 - Part throttle setting, anti-clockwise richens; 5k pot (RH pot), set to min (fully clockwise)
R4 - Idle setting, anti-clockwise richens; 5k pot (LH pot, nearest to wire coming off), set to max (fully anti-clockwise)
Transistor - BC184 (NPN bipolar)
How it works:
R4 provides an idle mixture adjustment (in addition to the one on the AFM). Manufacturers setting is fully anticlockwise (rich).
R3 provides an adjustment of off-idle mixture, i.e. throttle just open, switch closed. Manufacturers setting is fully clockwise (weak).
Set-up on my car:
CO was checked with standard ECU - idle set to 1.5% and off-idle is ~1.6% (about what you would expect). With Superchips ECU - off-idle is 12%+. This is much too rich and explains the very poor cruising fuel consumption and black exhaust pipe. However, jerkiness on opening/closing throttle at low speeds has almost gone!
Mods to Superchips ECU:
Changed R2 on Superchips PCB to 2k pot and adjusted for CO - lowest with with zero ohms. R2 shorted and original pot set for 1.6% off-idle. Resistance A-B is now ~12 ohms. Car runs well, cold start a bit lumpy for 10-20s but jerkiness on opening/closing throttle at low speeds is back!
What it does:
What the Superchips ECU appears to do is greatly richen the off-idle mixture so that when the throttle is initially opened the very rich mixture gives a poor response, and therefore cures the jerkiness. But there is a price!