Im trying to switch to a low barbell position on the squat from the high trap position. After trying for the first time today I have a few questions.

What exactly should be holding the bar in place? Is it my hands/arms, or is it my back? I find its more difficult to hold the bar this way when the weight gets heavy compared to just placing the bar on my traps. It seems my arms are working pretty hard to hold the weight up.
Should I just drop the weight until I get used to the new bar position?

Ive watched the Squat Rx vids and the Starting Strength book is on the way btw.