Pretty much the way any trigger works. Pressing the trigger moves the sear. The sear releases the hammer or striker, which hits the firing pin, firing the cartridge. Powered either by recoil, or by gas pressure from the cartridge, the action cycles, extracting the fired case, ejecting it, and loading another cartridge. A part called the disconnector prevents the hammer from falling a second time until the trigger is released, and then pulled a second time.
Bolt handle was provided as a back up in case the semi auto mechanism did not function. Which it frequently did not.
Your rifle may be a Winchester Model 490. If so, it should have a button type safety on the side of the trigger guard to the front of the guard. This pushes through from right to left to release the safety. Please contact Winchester through their website for an owner's manual for your rifle.
Semi auto air guns are driven by C02 gas. when the trigger is pulled some of the C02 not only fires the projectile but drives gears that get the next pellet or bb in line to fire the next projectile.
Automatic firearms work with a kind of "manual conveyor belt" mechanism. Tapping the trigger will fire a single round, akin to semi-auto fire. Holding the trigger activates a small set of gears that turn at a remarkable rate, ejecting an empty shell and replacing it with a fresh round at a continuous rate. Semi-auto doesn't have that same mechanism, or it is rendered inactive in semi-auto fire mode. The rate of fire is determined by how quickly multiple rounds are shot, or how quickly the gears turn. Examples of auto fire weapons are the SAW and M4A1, the latter having a semi-auto fire option.
Rear trigger is "set" trigger, by pulling it 1st, it makes the main trigger a "hair" or lighter trigger
Numrich Gun Parts Corp. Google Numrich Gun Parts. Very good to work with Jeff
Not recommended.
Automatic-rifle bullet are usually gas powered and the achieve great speeds by rapidly accelerating out of the rifle, usually in response to a trigger.
Receiver redesigned so that a full auto trigger group cannot be dropped in without extensive modification by someone qualified to do such work, redesigned trigger group which cannot accept an auto sear. If you're trying to figure out how to convert one to full auto, you need to stop right here - unless you possess a Type 7 FFL with the necessary Special Occupational Tax stamp to manufacture full auto firearms, attempting such a conversion is going to leave you with a non-functioning firearm (the most likely outcome), or 10 years in prison (if you somehow gain the know-how and make a successful conversion).
Semi Auto BB guns are designed to run on C02. When the trigger is pulled the pellet or BB is fired. The C02 also forces the cocking action back and reloads the next shot. So each time you pull the trigger the next shot is reloaded after the first shot is expelled. A Full Auto works the same way except the trigger only has to be held down. Re-cocking and firing keep recycling until all the projectiles are used up or the trigger is released. If you are talking about "Airsoft" then a battery, motor and spring do the same thing C02 does. The motor re-cocks the spring after each shot and reloads the plastic BB.
You take it to a trained gunsmith for evaluation and repair.
I would contact Diana directly at the link below and see if they will send you a diagram.