The mirror of a curve-steepener. You sell the front end and buy the long end in DV01-matched size, so you make money when long yields fall relative to short ones.
Flatteners are the classic hiking-cycle trade: the fomc pushes the 2-year up while the 10-year moves less because long-run growth and inflation expectations are unchanged. Taken far enough, the curve inverts.
Example: 2s10s at +90 bp. You put on a flattener with $2,000 of DV01 per basis point. The curve flattens to +20 bp over three months, producing 70 x $2,000 = $140,000, regardless of whether yields overall rose or fell.
Related: curve-steepener, twos-tens, bull-flattener, bear-flattener, inverted-yield-curve