Normally, horses have a set amount of gallops that can be used before you need to wait for them to recharge. Dismounting and remounting the horse immediately recharges all available gallops. Infinite Horse Stamina is a technique that dismounts and remounts a horse very quickly.
To perform the technique, you must have a shield and a bow equipped, as well as at least one arrow in your active quiver. When you want to refresh your horse’s gallops, hold target (ZL), the action button (A), and your bow button (ZR). With all three buttons held, tap the jump button (X). If done correctly, your gallops should be fully available again, despite not showing as available on the indicator.
If you fail the technique, it is recommended that you push the jump button (X) while in the air to pull out your Paraglider, which avoids damaging your shield on the landing.
This technique does not work when travelling downhill on your horse. You must be riding over flat ground or travelling uphill at the time that you press the jump button (X).