Logic/Sampliture "hybrid audio" question

[kp]

I gather that both Logic and the new (v8?) Samplitude have/will have what Samp are calling a "hybrid audio" engine, where input monitored tracks and soft synths are run at one latency and other tracks are run at much higher latency.

I'm confused by this, because as far as I understood, the buffer size was determined by the soundcard (ASIO buffer size) which is fixed by the user.

I'm trying to work out how this could possibly work - either the "fast" tracks or the "slow" tracks need to use this as their buffer size and the others use multiples or fractions (as applicable), and in both situations, my mind runs into inconsistencies.