A system that boosts priorities when threads block can have problems with threads that monopolize the system by blocking and unblocking very quickly. Without checks in place to prevent this, a thread could block and unblock so quickly that it essentially maintains the priority of its boosted state.
To prevent this kind of monopolization, the elapsed time since a thread was scheduled to run is totaled whenever the thread blocks. When the total reaches or exceeds the quantum period, the total is reset and the thread is blocked with an unboosted priority. Effectively, the thread is back in the runnable queue at its base priority.