Preset spillover is when the reverb of the current preset continues to decay out while the newly-recalled preset begins. This functionality requires two processors (or one processor with twice the bandwidth) to run the two reverbs in parallel while they overlap. The ILLUMINE Reverb does not have this functionality.
However, ILLUMINE will transition smoothly between presets when the two presets use the same effect algorithm and have the same effect options. In other words, only the five top-level parameters are different. In this case, the preset parameters transition smoothly to their new settings with no break in the effect.