Those aren't the needles I had the experience with. The ones I used had a spring loaded sleeve that popped out when the plunger was fully depressed. It went 360 degrees around the needle (the sharp part) and was about 1.5" long. The needles (the sharp part) were only about .25" inches long.

Once the sleeve was fully deployed it locked into place. So even though the plunger could still be operated, the needle wasn't able to stick anything.

I guess these are probably different than the UNVC syringe.