Why does the clamp QCPC open when you pull the knob?
It is normal for QCPC to open when you pull up the knob.
When you turn the knob to ON position, the shaft shown in red goes down and pushes down the balls by the spring force.
Then the balls hold the clamping pin.
If you pull up the knob after clamping, the shaft goes up because it is connected to the knob with a small screw.
Then the balls do not receive any pushing force and they become free and so unlocking the QCPC.