This takes advantage of the fact than when increment <= 1 (when not down- sampling), the scale factor is always 0. As a result, the scale and scale-phase deltas never contribute to the filtered output. Removing those multiply+add operations cuts half of the work done by the inner loop. Sounds that do need to down-sample (when played with a high pitch, or is 48khz on 44.1khz output, for example), still go through the normal bsinc process.