Then an upward head tilt seems to help the airborne molecules linger in the VNOs. A horse performing a flehmen is giving you an outward demonstration of a stimulated vomeronasal organ. Flehmen is not a uniquely equine behavior. Stallions are, by far, the most enthusiastic equine practitioners of the flehmen posture. In the presence of a mare in estrus, for example, they might flehmen several times an hour.
Geldings seem to flehmen the least.
In fact, it's theorized that the process of gelding seems to compromise a male's ability to detect and analyze pheromones, making him sexually ineffectual in more ways than one. But while sex pheromones are definitely the most likely flehmen trigger, they're not the only ones. We know that pheromones are present not only on horses themselves, but also on their bodily fluids and their manure. Like many animals, horses use droppings and urine to help advertise their sexual status and to mark their sexual territories. Mares in estrus will pass small amounts of urine several times an hour.