What a quick release is doing
An over-centre mechanism, and why it stays where you put it.
A quick-release lever is doing two separate jobs at once, using a cam-shaped lobe as a lever to squeeze a large clamping force out of a modest hand movement, and then carrying that clamping geometry past its own dead point so the clamp holds itself shut afterwards without a hand, a screw, or any friction lock needing to keep pressing on it.
What is actually happening
A bicycle wheel's quick-release skewer has a small, off-centre cam lobe built into the base of its lever, and as the lever swings closed, that lobe's distance from the pivot grows, steadily drawing the skewer's rod tighter and stretching the wheel's frame dropouts against it, exactly the way a cam earlier in this set was described as converting rotation into a chosen motion, here converting a swinging lever into a steadily increasing clamping force. Once the lever passes the point where the cam lobe is at its widest, continuing to close it actually starts easing the tension very slightly, which is the same dead point covered earlier in this set, and it is precisely this small amount of over-travel past the peak that locks the mechanism shut, since pulling the lever back open now has to first push the tension back up over that peak before it can release anything.
The ratchet-strap comparison
A cargo tie-down strap with a cam buckle works on the same two-part principle, a hand pressing the buckle's lever down draws the strap steadily tighter through a curved cam surface, converting a modest hand push into far more tension in the strap than the hand could apply by pulling directly, and once the lever is pressed fully closed, the strap's own pull on the buckle cannot push the lever back open because doing so would require the tension to increase even further first, exactly backwards from what the strap's own pull is actually trying to do. Nobody holding a tightened cam buckle needs to keep a hand on it to stop it releasing, which is the direct, everyday proof that the mechanism is holding through geometry rather than through grip, precisely the same trick a bicycle's quick-release skewer is performing at a smaller scale around a wheel axle.
Why the lever shape matters as much as the lock
The cam lobe's specific shape is doing real design work beyond simply providing somewhere for the lock to happen, since a poorly shaped lobe can either fail to build up enough clamping force before reaching its dead point, leaving the wheel loose even with the lever fully closed, or can build up so much force so quickly that closing the lever becomes uncomfortably hard to do by hand. A well-designed quick-release lobe is shaped so the clamping force rises smoothly across the lever's swing and only firms up noticeably in the final part of its travel, giving the person closing it clear, physical feedback exactly where the mechanism is heading toward its locked position, rather than a lever that feels loose right up until it suddenly locks with no warning.
The one number worth remembering
A quick-release lever closed through roughly the last quarter of its swing typically does most of the actual clamping work, since that is where the cam lobe's radius is changing fastest and building tension most rapidly, while the earlier part of the lever's travel mostly just takes up slack with comparatively little tension building at all. This uneven distribution is why a quick-release lever that feels easy for most of its swing and then noticeably firms up right near the end is behaving correctly rather than sticking, and a lever that never firms up at all across its whole travel is a genuine sign the mechanism has failed to build any real clamping force. A first-time user expecting a smooth, evenly resisted swing throughout can easily mistake this correct, uneven feel for a fault, which is worth knowing before ever judging a lever's condition purely by how it feels in the middle of its travel rather than at the very end.
Why this matters in practice
Understanding a quick-release as a lever building force and a dead point locking it explains both how to use one correctly, closing it until real resistance is genuinely felt rather than merely until the lever physically reaches its final position, and how to recognise a worn or poorly adjusted one, which fails to firm up properly even when fully closed because the cam's geometry no longer clears its intended dead point by enough margin. Both faults are invisible from the outside until the lever is actually tested by hand, which is exactly why checking a quick-release by feel, not merely by whether the lever has physically closed, matters every time one is used to hold something that genuinely needs to stay put. This same habit of judging a mechanism by the resistance it offers, rather than by whether a handle has simply reached the end of its travel, comes up again once this set reaches gearboxes and bolted joints, where an identical mistake, trusting a lever's position over the force it is actually delivering, causes exactly the same kind of quiet, invisible failure.
What this does not explain
A quick-release mechanism's clamping force depends entirely on how it was adjusted before closing, since the same lever geometry can produce a firm, reliable clamp or a dangerously loose one purely depending on how much the skewer's own length was set beforehand, a detail the lever's cam action has no way of correcting on its own. The lever supplies the mechanical advantage and the lock, it does not supply the judgement about whether the starting adjustment was actually correct in the first place, which is why a quick-release lever left to close easily with almost no resistance felt anywhere along its travel is not a sign of a gentle, well-tuned mechanism, it is a sign the skewer was left too loose to ever reach the part of its swing where the real clamping was supposed to happen.