Side-lying kick explained: learn rotation and balance for freestyle. Ear on your arm, 45 degrees, step by step with checkable milestones for each stage.
The side-lying kick is a freestyle kick where you lie on your side, one arm stretched forward, your ear resting on it. The honest answer to "Why am I lying sideways in the water?": because freestyle is not lying flat. Your body rolls with every stroke, and here you practise exactly that rolling in peace, without arms and breathing getting in the way.
Look at how much rolling there is: in a test with six swimmers at 200 m pace, they rolled 66 degrees while breathing and 57 degrees without breathing [1]. 90 degrees would be fully on the side, so you see that more rolling happens than most people think. The shoulders roll clearly more than the hips, and the buoyancy of the body contributes a lot to the rolling [2]. So you do not have to "make" the rotation, you mainly have to allow and control it.
Realistically: the basic position often works after two or three lengths with fins. Balance without fins usually takes a few sessions (experience, not a study value).
Your face is in the water, your eyes look down and slightly forward. This brings hips and heels up [3]. Pull your belly toward the surface and kick loosely from the hip, with the heels "bubbling" at the surface [3]. Milestone: you lie horizontally without your legs sagging.
Stretch one arm long in front, the other lies at your body. Turn hip and shoulder together to about 45 degrees to the side, head and body stay in one line [3]. 45 degrees is about half a roll between front and full side position. Your ear rests on the front arm. Milestone: 25 m with fins without tipping onto your front.
Make yourself long, from the fingertips to the toes [3]. The longer you lie, the easier it is to hold the side position. The front arm presses lightly downward, as if preparing to grip the water. Milestone: you manage 15 m without a pull buoy, your legs stay up.
A measurement on skilled crawl swimmers shows: during the leg movement a wave travels from the hip through the knee to the foot [4]. For you this means: the impulse comes from the hip, knee and foot follow loosely. If you only work in the knee, it gets stiff and noisy. Milestone: a calm, small kick with loose feet.
After 12 to 15 m you turn over your front onto the other side and stretch the other arm forward. Practise both sides equally long. Milestone: left and right feel almost the same.
Take the fins off. Now the rotation position has to hold without a buoyancy aid. The kick becomes stronger and smaller. Milestone: 25 m without fins and without sagging.
Leave the top arm lying along your side and tap your thigh with the hand now and then. If the hand arrives reliably without you twisting, you have the rotation under control. If you wobble, you usually pull too much from the neck.
In freestyle your body rolls with every stroke. At 200 m pace it is about 66 degrees while breathing [1]. The side position practises this rotation in isolation, without arms and breathing interfering.
Coaching drills recommend about 45 degrees, head and body stay in one line [3]. In freestyle itself you later roll further, but that happens almost by itself through buoyancy [2].
At the start they help a lot because they carry the legs [3]. By milestone 6 at the latest you should practise without them, otherwise the position stays dependent on the fins.
Hardly. At slow speeds the kick contributes almost nothing to forward movement [5]. The drill trains position and balance.
Catch-up Drill: Stroke Timing and Reach in Freestyle