The fourth movement is a characteristically rhythmic, energetic and propulsive Haydn finale. The music tends to focus on textures and moods rather than propulsive rock riffs. Musically, it incorporates a propulsive beat. Wong's drumming style has been described as "trancelike", "propulsive", and "explosive". Solar sails use radiation pressure in a way to achieve a desired propulsive force. Geared turbofans and further fan pressure ratio reductions will continue to improve propulsive efficiency. These also had a more vertical tail fin, used for a powerful propulsive stroke. The feet must be turned outwards during the propulsive part of the kick. It has been described as a "propulsive banger". However, at low speeds, these engines benefit from relatively high propulsive efficiency. Coincidentally, the bypass ratio grew to achieve higher propulsive efficiency and the fan diameter increased. Draco and SuperDraco combine the functions of a reaction control system and main propulsive engine. They combine the functions of both a reaction control system and a main propulsive engine. In December 2015, a Falcon 9 accomplished a propulsive vertical landing. However, these losses are more than made up by the improvement in propulsive efficiency.