obscure Patrick O'Brian Easter humor

So we're at church for Easter and the young priest is taking the censer around to swing it in the general direction of the people, releasing a rather modest amount of incense fumes. Hubby, who has been the censer-swinger himself in days gone by, recognizes immediately what's going on and bows (half the people around us don't know what to do and are imitating him).

After the priest goes by, hubby grins at me. "Do you know how I knew what to do?" Before I can answer, he says with a gleam in his eye...

"I smoked it!"