Questions about ULE and Priority Propagation

Hi all, i'm writing a document on schedulers in modern OS.. i've some
questions about the FreeBSD ULE scheduler..
First of all i couldnt find a good explanation for "priority
propagation", how does it work?? 
Second and most important,i'm reading Jeff Roberson documents about ULE,
he says there are 2 queues, current and next.. the scheduler picks up
from current queue and when its empty next and current r switched. This
guarantees fairness,  but later it's written that any intrrupt,
real-time and iteractive thread is inserted into current queue..given
this, i'm wondering if it could happen that a lot of real-time or
interective threads prevent queues swithing..perhaps i'm missing
something..
thank you in advance
Andrea