What power is Elrond referring to?

[davem]

Don't know if its relevant, but there is the passage in The Hunt for the Ring (UT):

My father nowhere explained the Ringwraiths’ fear of water....Thus of the rider (who was in fact Khamul of Dol Guldur*) seen on the far side of Bucklebury Ferry just after the hobbits had crossed it is said that ‘ he was well aware that the Ring had crossed the river; but the river was a barrier to his sense of movement’, & that the Nazgul would not touch the ‘elvish’ waters of Baranduin**.

*From notes recounting in detail the movements of the black Riders in the Shire it emerges that it was Khamul who came to Hobbiton & spoke to Gaffer Gamgee, who followed the hobbits along the road to stock, & who narrowly missed them at Bucklebury Ferry. The Rider who accompanied him, whom he summoned by cries on the ridge above Woodhall, & with whom he visited Farmer Maggot, was ‘his companion from Dol Guldur’. Of Khamul it is said here that he was the most ready of all the Nazgul, after the Black Captain himself, to percievethe presence of the Ring, but also the one whose power was most confused & diminished by daylight.

**[At the Ford of Bruinen only the Witch King & two others, with the lure of the Ring straight before them, had dared to enter the river; the others were driven into it by Glorfindel & Aragorn.