How does a woman wipe over her head covering (hijab), and is it obligatory to wipe over all of it, or only a part of it?

Some scholars, such as the Hanbalis, permit wiping over the khimar (head covering) for women, provided that the khimar is difficult to remove and is tied under the throat. They stipulate a duration for wiping, similar to the khuffayn (leather socks), and require it to be worn after achieving a state of ritual purity. The method of wiping involves wetting the hand and wiping over most of the khimar from the top. The majority of scholars do not permit wiping over the khimar during wudu' (ablution).