Auto retracting trimming knives have a slide button to push the blade out, once the button is released, the blade automatically springs back in to the handle.
These are very safe for when they will be put in a pocket, after all you could forget to retract a retracting knife which is not a possibility with this type of knife.
With traditional trapezoidal blade trimming knives, the blades are changed by unscrewing the two halves of the body and changing the blade.
Auto load trimming knives have a magazine of blades held in the body. Replacing the blade is simply a matter of pushing the release button when the blade is fully extended and taking the old blade out.
To reload a blade, the retract button is simply pulled all the way back and the new blade drops into position.
This makes for fast and efficient blade changes.
This is a safety feature of the Trapezoidal bladed trimming knives. Since the handles come in two halves to put a new blade in, theoretically the blade could slip out if the screw holding the handles together came loose. A potentially dangerous event.
To prevent this, the handle interlocks over where the blade sits.
This clearly shows that the blade is positively prevented from slipping out.