Props are often treated as a concession for beginners. Mechanically they do something more specific: they change the geometry of a shape, and geometry determines load.
Leverage depends on distance from the joint
The rotational demand at a joint depends on the weight involved and on how far that weight sits from the joint's axis. Distance is frequently the larger factor.
Extending an arm or leg further from the body increases that distance and raises the muscular effort required to hold the position, without adding any weight.
A prop that brings the floor closer, or that supports a limb partway along its length, shortens the effective distance and reduces the demand proportionally.
A block raises the ground rather than lowering the standard
In a forward fold or a standing side bend, placing a hand on a block rather than the floor changes where the trunk must reach.
That reduces the range demanded from the hips and spine, allowing the shape to be organized with the intended alignment rather than compensations elsewhere.
The compensations avoided are the point. Rounding the lower back to reach the floor loads the spine differently than hinging at the hips with support.
Muscles produce force best within a range of lengths
A muscle generates its greatest force at a middle range of lengths, where the contractile filaments overlap most effectively, and less at very short or very long lengths.
A position that places a muscle near the end of its range therefore combines high leverage demand with reduced capacity to meet it.
Props reposition the body so that supporting muscles work nearer their effective range, which is why a supported version can feel stable where the unsupported one felt precarious.
Straps extend reach without extending range
A strap allows a hand to connect to a foot when the two cannot meet directly, preserving the shape of a position without forcing the joint further than it will go.
It also permits sustained holds, since a grip maintained through a strap does not depend on holding an extreme position with muscular effort alone.
Bolsters serve a related function in restorative work, supporting body weight so that muscles can release rather than continuously stabilize a shape.
What props do not resolve
Reducing load does not make every position appropriate for every person, and some shapes place demands on the neck, shoulders or knees that support cannot remove.
Anyone practicing with a joint condition, a recent injury, or during pregnancy should have modifications set by a physical therapist or physician alongside an experienced teacher.
Understood mechanically, a prop is an adjustment to leverage, which makes it a tool for advanced practice as much as for early practice.