I do kettlebell swings outside, on the grass if I have the luxury, on concrete otherwise, and never on asphalt or synthetic material. The point is to feel the earth, to be under the sky, to breathe fresh air, and to join the weather, whatever it may be. I swing barefoot.
I find a clear shooting range for swings. The kettlebell is a solid chunk of iron and anything in its path, should I ever let it fly, will be either destroyed or damaged. If a fist can break a wall, a 70-pound kettlebell can crack a support beam. One of the commandments of gun safety is to know your target and what lies beyond your target. The same applies for kettlebell safety.
Doing swings barefoot builds balance. The toes spread and the arches stiffen in response to the swinging iron. As I hinge and the bell pulls me back, my feet keep my weight forward. The downward force presses through my arches into the ground just before the weight stops behind me. I pull the kettlebell back up and again the force drives down through my feet into the ground. This time it’s heavier on the heels. When my hips snap forward, and I straighten into a standing plank, the iron pulls straight ahead, wanting to fly. My feet spread against the ground to keep me still.
The kettlebell swing is a front-to-back movement, but it isn’t perfectly straight. There is some variation in pull from the left versus the right arm, and there is a slight twist of the torso. These variations project the kettlebell more to one side or the other, and this calls for resistance. When the iron strays a fraction of a degree to the side, your feet will instantly respond to keep you grounded. Keep your feet rooted in the ground and feel them push and twist and spread in response to the force of the swinging bell. You must be barefoot to fully benefit from the subtle changes in direction. This is how the swing develops your side-to-side balance.
Although the swing is a dynamic movement, with no resting position and no full stop between repetitions, the powerlifting principles of torque still apply. First, the feet are planted on the ground throughout each set. I line my feet almost parallel with only a slight outward angle, as I do in squats and deadlifts. The more I swing and squat and move with my feet pointing forward, the more flexible my ankles become. The ankles must have twist to translate force from the ground to the feet to the legs to the hips. If the feet are angled too wide, there will be no twist in the ankles. No twist, no torque.
In addition to keeping the feet planted and parallel, you must pull out on the knees to create torque. This will bring the force between your feet and the ground into your hips, giving you the power to snap them forward. It is similar to the torque during the squat and the same as the torque in the deadlift. Your feet can only grip the ground and generate this torque properly when they are bare.
Regular training on bare feet will build their musculature and arch. If you have never trained barefoot, and if you wear shoes that have even slightly elevated heels and cushion, you will feel a significant heel stretch at first. Most shoes drop in elevation from the heel to the toe and keep your ankle slightly flexed. This makes your heel and achilles stiff. The cushion in shoes disturbs foot mechanics and disrupts force transfer from ground by absorbing it. Even minimalist shoes like Vibrams will not replicate bare feet. The shape and curvature of the fabric and sole won’t allow all of your feet to contact the ground naturally. The grip of the rubber soles is too strong and interferes with the subtle mechanics your body would use when barefoot. Let your feet feel the ground and begin their development.
The skin of your feet will thicken and you will develop callouses. This builds readiness for rough surfaces. Find grass if you can, and train on concrete if you cannot find grass. Concrete is a semi-conductor of electricity because it holds water within its molecular structure. Concrete struck by lightning or activated by a strong enough electrical current will explode. This property is good for training. You will be earthing, or absorbing the electrical surface charge of the earth when your feet touch the ground. This also builds strength.
Do not be afraid of ground that is wet, hot, cold, or bumpy. Just remember that the kettlebell is iron and will rust if not dried after use. You can train in the rain, focusing more on grip and ensuring nothing is in the path of the kettlebell. Imperfect surfaces are also good, as you will learn to build stability on uneven ground. Try to have a reasonably level surface, so that you do not build imbalances within your body. Limit yourself to reasonable temperatures, as well. Progressively hotter or colder surfaces build toughness. Extreme heat or cold will damage your feet and knock you out of training.
Swing barefoot. You will build stability, healthy feet, strong posture, and toughness.