From Gibraltar on the Power Rack Extension Clamp: "These tube extender clamps are used for combining two shorter bars to make one long bar. Allowing you to connect two 1 ½" diameter rack tubes end to end.
"Please note: We ask that you don't use this clamp in a horizontal setting which means: don't use it to connect two horizontal bars together because if you start adding weight to the bar, wherever this clamp is, there will be a weak spot."
The video below demonstrates how to use many Gibraltar Clamps. At 6:57 he demonstrates how to use the Extension Clamp.