It's a tradition that is rumored to have started as a deal struck between St. Brigid and St. Patrick. (Though it's never been proven, and neither Brigid nor Patrick are saying anything on the matter.)
As for what it entails? Well.....think of it as "Sadie Hawkins Day" only on a different day and only occurring once every four years.
Or think of Stephen King's MISERY:
Of course the outcome need not be THAT extreme. Instead, it could just reflect the situation of many guys in a willing FLR.
But if it's the woman who pursues and proposes, she may have some other proposals in mind besides just marriage!
And once she has you, you may not be able to get away.
But then, if it's the right woman and the right sort of situation......you may not want to escape!