In that picture the lever handle has two holes at the bottom for pins or bearings. I believe if you look closely when the lever is fully lowered, the holes cross, i.e., the lower hole is located behind the upper hole which keeps the lever down. If so, it's likely something has worn so that doesn't happen. In an earlier picture there was a notch in a cam that caught the lever at the bottom. Those are the two most likely methods of holding the lever at the bottom. My Rancilio seems to use the overcenter method which I assume will be more common than a cam.