There is a difference in wording between a give way vessel and a vessel require not to impede the safe passe of another vessel.
A vessel required not to impede has to keep out of the way so as to not create risk of collision. In the event the situation would evolve to a risk of collision, even if the vessel required not to impede become the stand on vessel, it does not and remains with the obligation of not impeding. However, if the vessel of witch the passage of is not to be impeded is now also at risk of collision, he should comply with the colregs and therefore become either the give way or stand on. So the requirement of not impeding is one level above being the give way vessel.
In an overtaking situation, any other rule of part B section 1 and 2 are void, only rule 13 apply. That is if a vessel is required to to impeded fail to clear the way and it becomes an overtaking situation, the vessel of witch the passage is not to be impeded has to keep clear. There is no more impede stuff. Overtaking vessel has to keep clear no matter what.
This can arise in some strange situation like a vessel not under command overtaking another vessel, the vessel not under command has to keep clear... only rule 13 apply. Rule 18 start with "except where Rules 9, 10 and 13 otherwise require"