A duration-neutral position is constructed so that the total portfolio DV01 is zero — gains from one leg offset losses from the other when rates move in parallel. This isolates the trade's profit-and-loss to changes in the yield curve's shape rather than its level.
The construction process:
Example: a 2s/10s flattener with the 10-year having a DV01 of $820/million and the 2-year having a DV01 of $195/million requires a hedge ratio of 820/195 = 4.2x. For every $1 million of 10-year notes sold, approximately $4.2 million of 2-year notes must be bought.
Duration-neutral construction is essential because parallel shifts in the yield curve are far larger (in terms of P&L impact) than slope or curvature changes. Without duration-neutrality, a curve trade would be dominated by directional rate moves, swamping the intended spread exposure.
The Salomon Brothers yield curve primer emphasizes that virtually all professional yield curve trading is done on a duration-neutral basis. This applies to curve trades, barbell-bullet comparisons, and butterfly spreads.