On many car seats, you change the harness height by moving the straps into harness slots at various heights. To move the straps, you have to remove the end from a splitter plate beneath the car seat, take the straps out of the slots, and then re-thread them into the new set of harness slots before replacing the ends on the splitter plate.
A no-rethread harness means that you don't have to take the strap ends off the splitter plate and put them through the individual slots. you can still change the harness height, though. On no-rethread harness car seats, the harness height is often changed by pulling on levers, pushing buttons, or turning knobs. These devices raise and lower the harness so that it sits at the right place - at or below baby's shoulders for rear-facing, and at or above baby's shoulders for forward-facing.
The car seat shown here has a no-rethread harness that is operated by the orange button on top of the car seat shell. When you push the button, the entire head-wing portion of the car seat slides up and down, and the harness height moves up and down, as well.
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