How do lateral movements develop collection in dressage?

Lateral movements develop collection through a specific gymnastic mechanism: by requiring the horse's inside hind leg to step further under the horse's center of gravity while simultaneously carrying weight, they systematically strengthen the hindquarters' carrying capacity and teach the horse to shift its balance toward the haunches in the way that collection requires. The inside hind leg in a correctly performed lateral movement is the primary gymnastic target of the exercise — shoulder-in engages the inside hind under the body; travers and half-pass require the inside hind to step across and under while carrying weight; renvers develops the outside hind's carrying capacity. Each repetition of a lateral exercise is essentially a controlled moment of increased hindquarter loading that, accumulated over many training sessions, develops the muscular strength and neural patterns for carrying that collection requires. François Robichon de la Guérinière's insight that shoulder-in was the foundation of all lateral work was specifically an insight about how this movement prepared the horse for collection: the engagement it demands of the inside hind leg is precisely the engagement that collection requires, and shoulder-in develops this engagement more directly and more systematically than any other single exercise. The sequence of introducing lateral movements in the German training system — from leg yield through shoulder-in to travers, half-pass, and eventually pirouettes — reflects a carefully considered progression in which each exercise prepares the horse for the next level of hindquarter demand, building collection progressively through increasingly demanding lateral work. A horse that has been thoroughly developed through all stages of lateral work has typically developed the specific hindquarter strength and engagement patterns that genuine collection requires far more efficiently than one trained only through transitions and straightforward gymnastic exercises.