The full Swell under expression is one of the most powerful and thrilling effects of which the organ is capable. The crescendo that results from opening the expression pedal of the full Swell division can be dramatic.
Try this: engage all of the speaking stops in the Swell that contribute to a good full organ sound. Close the Swell expression pedal. Begin playing a full, slow-moving passage. Gradually open the expression pedal, allowing the full Swell to crescendo.