Injecting outboard audio into the Data input on the ACC socket on the rear of the rig, applies the outboard audio directly into the balanced modulator through a single transistor buffer, completely bypassing the ICOM's audio circuits. This allows you to apply EQ and limiting to the audio, and results in a superb sound for a rice box.
Scott is absouletly correct. I recently acquired one of these little IC-735s, as their AM modulation has been noted to be above average for a little rice box. I use an iBox to connect my audio rack to a particular radio I am using, which is mostly a FT-102. Using this same application with a cable from the iBox to the ACC of the IC-735 resulted in excellent audio with my RF power adjustment down to about 20 watts.
Thanks Scott for the tip and advice, it works like a charm!