From "Powerglide Transmission Handbook" by Carl H. Munroe:
p. 54: In addition, the oil pressure in the converter forces it forward. This action can be compared to a hydraulic jack. Converter charge pressure from the transmission enters the converter and pushes it forward toward the engine. In stock applications, the thin flexplate isolates this movement from the thrust bearing. Race applications often use heavy duty flexplates that will not "give"; in these instances, minimizing flow restrictions through the converter will maximize thrust bearing life. The problem is greatly aggravated should the cooler lines or cooler become restricted. Any restriction in the cooler or lines restricts fluid flow from the converter.