If you look at the rear of a normal magazine(the side away from those pointy things when loaded
), you will see a small maybe 3 mm "cut out" of the top rear of the mag(this "cut out" is where the bolt catch engages the follower)......When ammo is still in the magazine, the follower can not reach the "cut out" on the rear of the magazine to engage the bolt catch. As soon as the last round is chambered then fired( after the last spent case is ejected and the bolt is continuing rearward) the rear "cut out" of the magazine allows the follower to contacat a small "hook" on the bolt catch and pushes it(bolt catch) up into the path of the bolt carrier, stopping it. which in turn has lifted the top of the bolt release away from the exterior of the rifle's receiver, making it ready for a quick mag change.......Drop the mag, load another, and slap the bolt release on the top!
PS We found out that the USmarine is actually this guys friend who helped him with the name.....so only good natured ribbings pleaz.