If the trigger is held to the rear and not released during the firing process then the striker spring is fully compressed by the action of the slide.
As the trigger is released to the reset point, some of the compression (but not much) is released.
If the trigger is fully released then about half of the striker spring compression (by distance) is released.
So if you rack the slide on a Glock with your finger off the trigger (which is the way you should always do it, by the way
) then the striker spring is left about halfway compressed (in terms of compression distance) after that operation. The trigger action takes up the other half before the connector pushes the trigger bar down out of engagement with the striker and allows the pistol to fire.
If you're firing the Glock then the odds are you won't get the trigger released in the time it takes the slide to cycle so you'll have your finger on the trigger keeping the trigger bar in the rearward position. The action of the slide flexes the disconnector out of the way allowing the trigger bar to pop up and catch the striker as the slide comes back forward. At the point that the slide goes fully into battery, the striker spring is now fully compressed. However, normal operation requires that the trigger be released (at least to the reset point) before it can be pulled to fire the gun a second time.
The device under discussion pushes the rear of the trigger bar down independently of the connector when the slide is far enough forward that the pistol can be safely fired. However, the shooter must be holding the trigger to the rear or the trigger bar won't be far enough back to engage the device. So as soon as the shooter releases the trigger the pistol will stop firing.