I have a convertible so there is no support from the roof of the car to the top of the windshield and firewall area.
Cowl Shake occurs when the under windshield/firewall area flexes do to road motion transferred from mostly the front suspension but also the engine torque. By tying a mid-point of the front fenders to a center point of the firewall/cowl area with a Monte Carlo bar this flexing can be dramatically reduced.
Its simply a triangulated brace that supports the area just above the shock tower between themselves and the firewall/cowl. I feel it as a dash structure rattle/vibration between the front door mounting pillars.
It helps me to visualize our uni-body chassis based cars as flexing much like an empty shoebox. If you hold the box from both ends and lightly twist in opposite directions, the box has very little resistance to torsional flexing.
