Twist targets the shape of the curve rather than the quantity of reserves. By taking duration out of private hands and giving back bills, it compresses term-premium at the long end while leaving bank-reserves roughly unchanged.
It is attractive when a central bank wants to ease financial conditions without the political and plumbing costs of outright balance sheet expansion. The mechanism is the same portfolio balance channel that makes quantitative-easing work.
Example: the Fed sells $400 billion of holdings under three years and buys $400 billion of 6 to 30 year paper. The balance sheet total is unchanged, the 10-year falls around 15 basis points, and the curve flattens.
Related: quantitative-easing, term-premium, curve-flattener, balance-sheet-runoff, duration