The Bobstay
The bowsprit of a sailing ship extends forward from the bow, carrying the forestay and jib sails. Every sail set on the bowsprit pulls upward — the wind in the jib lifts the bowsprit's tip. Without a counterforce, the bowsprit would rise until the rigging slackened and the jib collapsed. The bobstay is a chain or rod running from the tip of the bowsprit down to the waterline at the stem. It pulls the bowsprit down against the upward force of the sails. The bobstay carries no sail. It exists solely to prevent the thing that carries sails from moving out of position.
Guy wires on radio towers serve the same function. The tower carries the antenna and the wind load. The guys carry nothing themselves — they radiate outward and downward from the tower to anchors in the ground, pulling the tower toward vertical when wind pushes it sideways. A guyed tower can be much taller and lighter than a freestanding one because the guys supply the lateral resistance that the tower's own cross-section cannot. The guys do no broadcasting. They do not even touch the antenna. They stabilize the structure that holds the thing that matters.
Tendons in the human body play the bobstay role for the skeleton. The Achilles tendon connects the calf muscle to the heel bone, pulling the heel upward to push the forefoot down. But the tendon also prevents the ankle from collapsing forward under the body's weight when standing. It is simultaneously an active mover and a passive restraint, depending on what the foot is doing. When you stand still, the Achilles tendon is a bobstay — pulling the calcaneus into position against forces that would rotate the ankle forward. The same structure that creates motion in one context prevents unwanted motion in another.
The bobstay is the counter-tension that keeps a loaded member in its working position. The bowsprit must stay level. The tower must stay vertical. The heel must stay planted. In each case, the primary structure is being displaced by the very forces it was designed to engage — wind in sails, wind on tower, gravity on body. The bobstay does not carry the working load. It carries the reaction to the working load, pulling in the direction no one looks because it opposes the direction everything is pulling.