The sync plug that fits Vivitar flashes like the 283 and the 285 is not just a regular 1/8th" audio jack- it's a strange little proprietary jack. Yes, some other similar plugs might make the connection work sometimes, but they won't fit as well, and will make the flash sync even more unreliable. In any case, I don't think the 550 had a port for a sync cord. If I'm right, the only sync connection it has is the hotshoe mount, which means you'll need a female-hotshoe-to-cord adapter of some sort.
As for how the thyristor technology works, which hasn't really been covered here: the flash unit puts out light, which is reflected off the subject and comes back into the sensor. When the thryistor detects enough light coming back into the sensor to provide correct exposure at the aperture/ISO speed setting the unit is set for, it quenches the flash output. This all happens literally at the speed of light, in real time during the exposure. Provided that the sensor isn't blocked, and can see light reflected off the subject, it is surprisingly accurate.