Barbara Seagram

“What about very strong balanced hands? If you have enough to make a slam try with a relatively balanced hand containing a five-card major, you can transfer at the two-level and then bid 4NT: This 4NT is not Blackwood: it has the same quantitative meaning as it does when you bid it directly over the 1NT opening except that now you have shown your five-card major. Obviously, instead of simply choosing between 4NT and 6NT, opener must now also consider 5♠ and 6♠ too. If you are even stronger than this — a five-card major and the values to raise to 6NT — you transfer to your major at the two-level and bid 5NT. This is forcing — opener must choose between 6NT and slam in your major.”
― Barbara Seagram, 25 Bridge Conventions You Should Know - Part 1: Learn These First