It is a button on the mag release that can only be operated by a bullet tip. It is supposed to prevent you from quickly changing a magazine. On an AR it can be removed and replaced in about 2 minutes with no tools.
The dreaded pistol grip can also be installed in a couple of minutes. If a criminal wanted to, they could install a pistol grip, commit a crime, and take off the pistol grip. The stupid law only affects the law-abiding.
The magazine limitations are similarly worthless. Even aside from the fact that millions of such magazines exist outside of Kalifornia, the technology of a gun magazine is practically 16th or 17th century. Fold some sheet metal into a box shape, cast a follower, and make a spring. There will never be a day in the future of the world where criminals are effectively denied use of a normal capacity magazine.
The only real effect of these laws is to drive law-abiding citizens, some of whom are already armed to the teeth, to purchase even more armament.
A guy I know stored several rifles out of state when the original "assault weapon" laws were passed. Within four years he had equivalent rifles back in hand through the workarounds created by the gun community and through the courts finding some of the laws vague and unenforceable.