I've noticed that when slowly triggering a note, there is one brief, higher-pitched tone played prior to the sound of all of the drawbars. It's not percussion...it just sounds like one of the higher drawbars is being triggered prior to the others. I didn't have a lot of time to play around with it, but it doesn't sound correct to me.
As Mighty Ferguson pointed out, this is a simulation of the key contacts. On a real organ, the quicker you press a key, the quicker all contacts are made (there are 9 under each key). But if you play very softly, you can actually get all frequencies to play one by one as each contact is made. We modeled that with velocity, so it's not entirely correct as velocity does not equal speed of key struck, but it's a good approximation.