Abstract: A method is provided for using a rotatable insertion horn to open an envelope prior to insertion of documents. A pair of insertion horns is positioned at the sides of an envelope inserting station. An envelope is fed into the envelope inserting station with its flap open. The collation of documents is pushed into the open end of the envelope. The insertion horns are positioned in an initial position that is fully outside the envelope, prior to arrival of the collation. The insertion horns are controlled to simultaneously rotate into the envelope as the collation passes between the insertion horns. Preferably, rotation of the insertion horns into the envelope does not begin until a lead edge of the collation is in a region between the insertion horns. This facilitates insertion of the collation without catching on an upstream edge of the insertion horns.