The wonderful things you find in dictionaries. This from Grose's Dictionary of the Vulgar Tongue (1811):
[i.e. a Welch rare-bit] Bread and cheese toasted. See Rabbit.—The Welch are said to be so remarkably fond of cheese, that in cases of difficulty their midwives apply a piece of toasted cheese to the janua vita to attract and entice the young Taffy, who on smelling it makes most vigorous efforts to come forth.
Incidentally, it is, or was, Welsh rabbit before it was Welsh rare-bit. The rather unkind idea was that Welsh things were poor substitutes*. For example, a Welsh carpet was a pattern painted, or stained, onto a brick floor; a Welsh diamond is a rock crystal; and a Welsh comb is your fingers.
Do any Welsh midwives read this blog? Is this still the standard method?
Come on out
*But Ian Rush was great.
P.S. Janua vita[e] means the gate of life. Its meaning should be obvious; if it's not, consult your parents. The usual Latin phrase is Mors janua vitae, meaning Death is the gate to [everlasting] life. It's therefore the same root as January and janitor and Janus.