Weight aids explained simply: how seat and pelvis show direction and tempo, what measurements actually show and which mistakes to avoid as a beginner.
Can you really steer with your seat? The honest answer: yes, but much more quietly than you think. Weight aids are not leaning movements but tiny shifts of your weight in the saddle. The FEI describes aids as an interplay of seat, legs and hands and requires that all movements succeed with barely perceptible aids [1].
How small these shifts really are was shown by a Dutch measurement study with 11 riders. Riding straight ahead, the mean total force under the saddle was 671 newtons; in travers it was 726 newtons. The inner half of the saddle carried 468 newtons in travers instead of 425 newtons, almost 43 newtons more [2]. That is roughly the weight of four bags of flour more on one side. It sounds tiny, and it is. The study does not say what the horse perceives of it. It only provides the measurements.
The bigger problem is often crookedness: in a test with 147 riders on a wooden horse, 99.3 % loaded the two stirrups unevenly while sitting [3]. So weight aids do not begin with shifting, they begin with sitting straight.
Sit deep in the centre of the saddle. According to the FEI the rider sits supple and deep in the centre and absorbs the movement with loins and hips [1]. Your centre of mass should lie almost vertically above the horse's, roughly at the level of the 14th thoracic vertebra [5]. Milestone: Both seat bones carry equally, you feel neither side "hanging".
At a standstill, alternately load the left and the right seat bone a touch more. You move so little that a spectator can barely see it. Milestone: Your horse notices (ear, head) without stepping away. This is a feel from practice, not a measured value.
Ride a big line and load the inside seat bone slightly more, with the shoulder axis staying parallel to the horse's shoulders. That way the weight works together with the leg aid. In travers the measured force rose in the inner part of the saddle and in the outside leg [2]. You will feel this connection later, for now the idea is enough.
Sitting tall and deeper in the saddle makes you "bigger" and can slow the tempo. A study of riders at the walk shows that pelvic motion and horse motion can be coupled differently in time [6]. Whether a particular pelvis technique helps or harms the horse is still open [6]. So keep your pelvis quiet and following rather than pushing forward.
Weight only works in combination. The FEI describes the half-halt as an almost simultaneous, coordinated action of seat, legs and hands [1]. So never practise weight aids alone, always the interplay with leg and rein. Milestone: Your horse reacts to a combination of aids without you getting louder.
Weight aids do not replace supervised training. As a beginner practise them only with an instructor, at the walk and in the arena. If your horse suddenly jumps forward or steps away at small shifts, that is a case for your instructor, not for "more weight". Mind your own body too: crookedness is normal, but it is discussed as a possible trigger of back pain in rider and horse [3]. That is a possibility, not a proven fate. If you feel pain in your back or pelvis, stop and talk to your doctor.
Do the mirror test without a horse: sit on a gym ball in front of a mirror and alternately shift weight to the left and right seat bone. If you do not tilt your upper body along, you have the feeling you need on the horse. This is training practice, not a study recommendation.
Barely noticeably. In a measurement study it was about 43 newtons more pressure on the inner half of the saddle in travers [2]. If a spectator can see your shift, it is too big.
Very likely a little. In a test of 147 riders, 99.3 % loaded the stirrups unevenly while sitting [3]. Have yourself filmed or watched from behind.
No. The FEI describes the aids as an interplay of seat, legs and hands [1]. Weight reinforces but replaces nothing.
Because even a 5 cm difference changed the movement of the horse's spine [4]. That does not mean your horse is harmed, but that crookedness is felt by the horse.
In principle yes, but as a beginner first in the arena with an instructor. Out on a hack extra stimuli act on the horse; staying calm matters more there than fine weight aids.
Leg Aids: Driving, Yielding, Containing Without Constant Pressure