Basically one main reason you need to stop the world in common GC's right now (and thus cause pauses) is because if you want to compact memory a GC thread has no idea if other threads reads/writes to an object while moving leading to worst case corrupt/lost writes.

What Azul does is to make the pages in question invalid for reading/writing so any access to them would page-fault and then proceeds to move the objects without worrying about other threads. Since the hardware provides the page-fault "for-free" this protection doesn't cost anything in terms of runtime performance in the common case (compared to the new ZGC made by Oracle that has read-write barriers that steals some mutator performance), worst case if another thread tries to read/write memory that is being moved a specialized page-fault handler detects if this was an page that was in motion and can then do a slower read-write but ONLY in the seldom cases where this occurs (compared to all the time of f.ex. ZGC)