What is the Bell Tension

My Camry makes a slight shimmy sound when first stepping on the gas, a mechanic checked it out and said it could be 1 of 2 things, a loose belt or the Bell tension. He replaced the belt first because he said it would be the least expensive thing to do and that might fix it. Unfortunately it did not although it sounds a little better. He said that I would have to call the dealership for the Bell Tension and either they or he could put it in. He didn't know what it would cost?