Wildcard:
What does the little magnet do? My didn't come with one, I was just wondering....Thanks
As Gilligan said, it controls the Advanced Cancel Circuit. ACC was designed for cameras that emit a preflash.
Advanced Cancel Circuitry (ACC) increases camera battery life and also speeds up the camera's flash recycle time.
The default setting on a D 180 or D 2000 strobe has ACC activated. In other words you do not have to take any actions to activate the system. You only have to take an action, in this case installing a magnet, to deactivate the system. ACC was designed for cameras that emit a preflash. A preflash aids the camera in determining exposure. If your camera emits a single preflash, then it has the ability to take advantage of Advanced Cancel Circuitry. If however your camera does not emit a preflash, you must shut off the ACC. To shut off ACC, you must install the magnet into the strobe.
As you read this explanation of ACC, keep in mind that the camera's onboard flash is blocked from influencing the actual composition. A digicam's onboard flash's responsibility in this situation is only in sending an optical signal to fire the external Inon strobe.
ACC "magnifies" the preflash.
When ACC is activated, the Inon strobe "replaces" the relatively "less intense preflash pulse" (which is blocked from influencing the composition) with an artificially "magnified blast" of preflash light of its own. The camera sensor instead detects the "magnified blast" and calculates the exposure with that information. The camera is fooled into thinking that there is much more light present in the composition. Since the camera now feels there's a significant amount of light present due to the feedback it received from its sensor, it commands its own onboard flash to emit a much smaller amount of strobe energy for the main shutter event.
Less onboard flash energy emitted, less camera battery energy used. Shorter onboard flash duration, quicker recycle time for camera's flash to recharge equals quicker turnaround time for next image capture.
When you set the strobe to S-TTL, the camera is responsible for controlling strobe output. In this case, the Advanced Cancel Circuitry is automatically deactivated so as not to confuse the calculations.