a vehicle restraint employs a 4 bar linkage to lift a restraining member to a position that engages a vehicle thus restraining it at a loading dock . the linkage is upwardly biased to hold the unit in an engaging position and may be retracted by using any of a number of mechanical techniques such as hydraulic electric or pneumatic power . the 4 bar linkage transfers all loading on the unit from the icc to the frame thus isolating any driven members from overloads . a secondary hook mounted to the 4 bar linkage is pivotally spring loaded to limit vehicle travel away from the dock and to extend the capture range of the device in the case of icc &# 39 ; s bars that are damaged or mounted at non - standard positions at the rear of the vehicle .