Are you referring to the plastic device that comes with gun?
If so, place it over the magazine, put the magazine on a firm surface and depress the top of the loader with your thumb. Insert a round until you touch the inner lip of the loader, then release thumb pressure while advancing the round all the way in.
With a little practice, you can load a 17 round mag in seconds. It is a small learning curve, but you will be pleasantly surprised when you figure it out.
They are inexpensive, simple, small, fit into a shirt pocket, and work unbelievably well. Best of all it has no moving parts. Beats hauling around these other contraptions that you almost always forget when going to the range. It will load ANY magazine regardless of size, shape, or compacity. Bill T.
Gee, if you'd posted that before I bought the Butler Creek, 'UpLULA' pistol magazine loader, used it for a few weeks, and then went and threw my old HKS magazine loader away, I would have sent the thing to you for free!
Ok--i bought the product and it arrived in great time--but it is useless. I am a law officer who needs to load 13 rounds into a Glock 21C mag. This thumb device is of absolutely no use in this instance....sorry