Consider the 2/1 auction that begins
1S - 2D
2S
Depending on your style opener could be showing six spades, or simply be temporizing with a hand unsuited for 2NT and/or without a second suit. How often does this ambiguity derail the auction for want of a perfect 2NT rebid?
John Schuler sensed that it was often premature for opener to rebid notrump naturally at this point in the auction and could wrong-side the contract. His method uses the 2S rebid to capture (most) hands with exactly five spades (many of which would have rebid 2NT previously) and using 2NT to show those hands with six or more spades (that are less likely to end up playing in notrump.) This method gives the partnership time to explore before making a notrump suggestion while disambiguating opener’s suit length early on. (Note, the Schuler Shift similarly applies after opening 1H, and after any 2/1 response.)