env 2 sustain and vcfs on korg ms-20

[numbertalk]

Could someone explain to me how the sustain setting works exactly on Envelope 2 as it relates to the filter cutoff frequency of the 2 VCFs on the Korg MS-20? It doesn't seem to work exactly like a normal ASDR EG - when the sustain is fully up at 10 the envelope does not seem to sweep the filter at all, regardless of the settings of the attack, decay or release. However, when you turn the sustain down, especially fully to 0, you get the full effect of the envelope on the cutoff frequency. It seems to work normally as it effects the VCA since with the sustain on 10 you get full volume. So, I get that it almost seems reversed for some reason with the signal it sends to the VCFs, but could someone explain exactly what's going on underneath and why it was designed this way? Thanks!