We’re taught at school that warm air rises, and as cold air is heavier than warm air, it falls to the ground. When opening both sashes, the power of this natural physics lesson takes over. Some of the warm air at the top of the room escapes through the gap at the top of the window, as it does so it allows cooler air to enter the room at the lower level, being cooler and heavier this falls to the floor, pushing more of the warmer air higher, and starting a free, natural, zero cost air conditioning system!
This natural ventilation happens far quicker than any other window style with equivalent opening areas. Unlike casement windows which actually trap stale air, moisture and spores in the top half of a room, sash windows typically exceed 4 complete room air changes, every hour.