There is a video of it on YouTube. But basically there is an access hole that you use a 3mm allen (hex) key. You'll need to line up the access hole in the Cone. It's easier to see if the disc cap is off. Like in the picture below.
The Allen key will turn an angled piston that pushes the cone to lock the clamp. Tightening the bolt and slot nut, may help, but it's a very coarse adjustment. The Allen key is a finer adjustment.