Floating grey right and left screw has prismatic joint with orange crank. Pink right and left hand nuts control motions of green clamping bars.
Turn the orange crank clockwise to clamp yellow workpiece.
Red springs help to maintain contacts between the nuts and clamping bars.
The clamping bars may not move synchonically but at the end they clamp tightly the workpiece that is centered on a V-block.
If the R&L screw is not installed free in axial direction, maybe only one green clamping bar contacts the workpiece.