The most prominent feature of the IR (Internal Rotation) type is that the legs(femur) visibly rotate inwards and the knee caps also face inwards. Although there may be individual differences, it is usually accompanied by anterior pelvic tilts and back knees (a body shape in which the knee is located behind the hip joint, as shown on the left side of the picture below).

Because the center of the body leans forward, the arch of the foot is likely to collapse, and the shin bone (Tibia) is likely to rotate inwards too. Overall, the legs are inwardly rotated, so sitting in a W position like the image below can feel more comfortable than sitting in a cross-legged position. You can often see young girls sitting in this W position which is why it isn't surprising that the IR type tends to be observed a lot more in relatively young age groups.