To CONQUERER23, if your jaw hits your cerebrum or any other part of your brain, your skull must've been crushed. No human can generate enough force to do that nor can you really do anything about it.
To the OP, if you mean by stretching, the act of stretching the muscles around your neck to limber/loosen up, then no, that won't affect your ability to "resist" a KO. Muscles are built to stretch and contract on demand. Making them limber does not affect their ability to contract unless it's over done and damages the muscle. Preventing a KO is about bracing your head so your brain does not slosh around in its cage.
If you mean by stretching, that your neck is stretched out, then yeah, that makes you more susceptible to a KO. If your neck is stretched out, the neck muscles are supporting the head at their max capacity. More force means your neck muscles are overloaded and cannot brace your head. This can result in bad things happening including a KO.