Now let me explain a few things about how your injector and its cuircitry works.
First off a TBI injector is refered to as a 40ppm or continuous flow. What this means is that the injector is capable of roughly 40 cycles per second with the ability to constantly stay open.
Now for how the circuitry works. In simplistic terms, everytime the rotor hits a pin on the cap, there is a reference signal sent to the coil and to the ecm. This signal tell the ignition system when to fire, and tell the computer when to fire the injector.
Also this means the ignition system and the fuel delivery can function independantly. However the injector is its own entity, and is computer controlled. If their is no signal going back to the computer, the injector will NOT function. However the ignition will.
The signal is determined at the distributor. It comprises of the coil, ignition module, pick up coil and all the control wires.
Now for the injector itself. The injector is setup very similiar in the aspects of electricity, very similiar and nearly identical to the tail/signal/brake light systems, only found on S10s'.
There is always two wires for every injector, one that appears as positive (Constant Flow) and one that appears as Negative (Pulsed postive flow).
What you see as a ground, is not actually a ground. Its a wire that is dual purpose.
When you key up, the Positive ALWAYS has positive power. An so long as your not cranking, the negative is negative. Injector is off.
When cranking, the negative actually is used as a pulse wire. The computer will pulse a positive charge down the wire. This causes the circuit to negate itself, and the injector opens. A continuous flow down this wire, holds the injector open.
A positive charged wire that connects to another positive charge wire, actually negates both positive charges, and creates and electron impass. Resulting in zero electical flow. IE It shuts the circuit off.
Hence why when you check the injector, you hit BOTH SIDES with a positive charge. Hence the jumper method.
this is how your tail/signal/brake lights works, also