Answer: Some women have the custom of circling the candles with their hands -as if to motion with the hand that the blessing that follows pertains to these candles (just as we hold the food we eat, before we bless upon them, see Mishna Berura 206:17).

However, from the Code of Jewish Law it would appear that the custom is just to put the hand in front of the candles, so as not to see them before the blessing. Usually we make a blessing and then do the thing (like eat challah or blow the shofar). But when lighting Shabbat candles, once we make the blessing it would be considered Shabbat and then too late to light fire, which is prohibited on Shabbat. So in this case, we light first, but cover our eyes until after the blessing, thus recreating the "lighting" after the blessing. Make sense? (source: Rm"a [quoting the Maharil] in Shulchan Aruch, Orach Chaim 263:5)