It is actually pretty simple; if you're in the left lane on a multi-lane road, there is faster traffic coming up behind you and you can safely yield right of way by changing lanes to the right, you are obligated to do so, without regard for your speed or the speed of the traffic approaching from the rear. If (on the other hand) you are already passing traffic on your right at a reasonable speed (ie, you aren't pokeying along), you have no problems, because you're already passing - you're in the right (on the left). Of course, when your pass is done, and you can safely change lanes to the right and thus allow passing traffic to go by you, you should do so. What you cannot do is either (1) stay in the left lane and force traffic to pass you on the right, or (2) stay in the left lane and pace traffic to your right, thus effectively precluding passes of any kind - and why would you want to? It just creates congestion and increases danger for everyone behind you and around you.
In congested traffic - typical urban freeway with all lanes at capacity - there is no practical way for this law to have effect, because no one is passing anyone.
Really, it's a common-sense standard, and (again) there is no way to legislate every possible permutation of traffic interaction.