The problem with that concept comes from the fact that it is not enough to use just your inside leg without paying attention to many other factors. First of all, "inside leg" is not just your calf, it is your whole leg starting from the hip and the seatbone. It acts as a pillar, creating a barrier around which your horse bends. Second, it is not just outside rein, it is the outside side of your whole body that have to "catch" your horse's energy as he gives to your inside aids. Otherwise, he will "spill" himself over to the outside. Your outside arm connected to your hip and your outside leg, again including the hip and the seatbone, must catch and direct your horse. If you do it right you will get a feeling that your outside aids control your horse, his balance and direction. You will get a feeling that your horse fills the outside side of his body under you and it is very comfortable to ride. Shoulder-in gives you the best test for this. If you do your job well and gather all energy together and keep it all balanced you will feel like riding an airplane which is taking off. Your horse will stay cadenced, forward, his shoulders going up with every step. You will never forget that...