It's very much about developing a feel for what is correct and what is not. Your leg should be positioned underneath your hips, so that your at the center of your balance. Then you should be using the inside of your calf to squeeze at the horse's side. It is also accompanied by your seat/weight. For example, if your turning into a circle, your upper body (pivoting at your waist) will be turning towards the direction of travel (controlling shoulders with rein) and this will also cause your hips to turn + your legs to go in a "turning position" (inside leg on to support the horse, outside leg on to help turn). You can see this occurring in the video above at 1:04 to 1:13 when the rider is riding through the corner. Although, It is subtle in that video because that horse is most likely very sensitive to the riders body signals.
The main idea of it is that horses generally move AWAY from leg pressure, but it isn't just leg that helps the horse turn. EYES and SEAT/weight create direction, while the legs manipulate shape in a turn. Like SteadyOn mentioned, where your leg is placed also matters.
Behind the girth= haunches
Infront of the girth=shoulders
Left leg only infront of the girth= horse moves shoulders to the right
Right leg in front of girth= horse moves shoulders to left
So, If you were turning a square turn to the right, you'd first look, then pivot your upper body to the right, open your inside hip to allow the horse to turn in, support the horse with your inside calf and close your left leg to follow through with the turn. The degree on how much leg, seat, etc is used depends on how responsive that horse is. Use as much as you need until you get an adequate response.