This operation is best carried out off-camera, and with the holder flat in your hand. Not only is this easier, but also it prevents the filter being dropped.
Push the clip hard with the ball of your thumb – on one side only.
At the same time steady the unit with your fingers on the other clip.
You do not need to squeeze both clips.
Once disengaged, the Polariser will come away from the holder easily. The action is quite stiff, as this prevents accidental separation when the filter is in use.