A horse with a high stifle has a somewhat triangular-shaped thigh area. A horse with a low stifle has a square thigh area. Pictures (A=high stifle, B=low stifle):
The stifle should be positioned well foreward, nearly in direct line with the hip bone above it. If the stifle is too far back, the thigh will be too short and more V-shaped than square when viewed from the side. If the stifle has good muscling, this will be the widest part of the hindquarters when viewed from the rear; the horse will be slightly wider at the stifle than at the hips. The muscles over the stifles should be adequate, making this area appear fairly rectangular when viewed from the side or rear. A horse should be wide through the stifles, not narrow, because the stifles need to clear the belly when the hind legs move forward. Narrow stifles mean the horse will have less freedom of movement. If a horse is too narrow at the stifles, his pelvis may also be narrow and his thigh bones may angle out as much as they should when they extend to meet the stifles.
Here's a few pix of the stifles:
Ideally, the femer and tibia are approximately the same length, resulting in a relativelt low stifle. This allows room for longer thigh muscles, which enable the horse to generategreat power and speed with a longer stride. If the femer us too short, the stifle is too high (sitting above the sheath on a male horse or the udder on a mare) and thigh muscles are too short for a long stride. Short femur/high stifle conformation is better suited for short, quick strides of sprinting or for pulling (draft horse). If the femur is long, the stifle will be at the same level or slightly below the sheath or udder. The ideal angles create an equilateral triange between the point of the hip, the point of the butt, and the stifle.
Having a low stifle is a great advantage. Your horse will have longer strides, will have enough scope due to a low stifle placement, and yet won't rock the rider out fo the saddle.
Does that clear things up somewhat?