Why is putting weight in the stirrups a good idea before mounting a young horse for the first few times?

Putting weight progressively into the stirrup before ever swinging a leg over a young horse is one of the most sensible and effective preparation steps available to a trainer, and it is a step that experienced horsemen use consistently precisely because it eliminates one of the most startling aspects of first mounting — the sudden arrival of significant weight shifting the saddle and pressing into the horse's back — by introducing that sensation gradually and in stages the horse can process and accept before a rider is fully committed to the saddle. The horse's first experience of weight in the stirrup is fundamentally different from anything in his prior training. Ground work, longe work, and saddle introduction have all been conducted with the saddle sitting balanced and still on the horse's back. The moment a rider places a foot in the stirrup and begins to load it, the saddle shifts slightly to one side, the tree presses unevenly on the back, and the horse feels a novel lateral pressure that he has not encountered before. For a horse that has not been prepared for this sensation, the shifting saddle can trigger a startle response — a jump sideways, a crow-hop, or a spin — at exactly the moment the rider is most vulnerable, balanced on one foot with the other in the stirrup and nothing to grab hold of. The correct approach is to introduce stirrup weight in progressive stages before any attempt to mount. Begin by standing alongside the horse at the shoulder, placing a hand on the saddle, and pressing down firmly to shift the tree and create the same lateral pressure the stirrup weight will produce. The horse that stands quietly while the saddle is pushed and rocked from both sides has already processed the feeling of asymmetrical saddle pressure and found it unthreatening — a valuable first step. Next, stand at the mounting position, place the foot in the stirrup, and press down with steadily increasing weight while keeping both feet on the ground. The horse feels the stirrup leather stretching, the saddle tilting, and the tree shifting, but the trainer remains fully grounded and in complete control of how much weight is applied and how quickly. From this position, progress to lifting the body slightly so more weight transfers into the stirrup while the trainer can still return immediately to the ground. Watch the horse's response — ears, breathing, muscle tension across the back and hindquarters — and adjust the pace of progression based on what the horse shows. A horse that remains still and relaxed with full stirrup weight applied can progress to the trainer standing in the stirrup with the body raised alongside the saddle, which introduces the sensation of weight elevated alongside the horse's back without the full commitment of swinging a leg over. This standing-in-the-stirrup stage is invaluable because it allows the trainer to lay over the saddle from both sides, rub and pat the horse across the back and croup while elevated, and allow the horse to feel and see a human body above and beside him in a controlled way. Many horses that would have spooked at the sudden appearance of a rider sitting upright above them accept the same rider calmly after several sessions of progressive stirrup loading, because the height, the weight, and the visual presence of a person above the saddle have all been introduced separately before they are combined in a single full mounting. The entire stirrup loading process from first pressure to quiet standing-in-stirrup typically takes no more than two or three short sessions for a well-prepared horse, and the investment pays back immediately in the quality and safety of the first actual mounting. A horse that has been through this process stands quietly at the mounting block, accepts the rider swinging over without tension, and begins his first ridden steps already familiar with what weight on his back feels like — which is exactly the calm, confident start that sets the tone for all of the ridden training that follows.