Either standing up or sitting down in front of a mirror, flex the elbow to the end and raise the arm by flexing the shoulder, while using the opposite hand to push the elbow backward.
There is a tendency to rest the helping arm on the head as a kind of lever, which could force the cervical vertebrae. While this help could be useful if it is done correctly care must be taken not to adopt poor postures with the neck.
It is imperative to flex the elbow maximally (hence the hyperflexion in the name), yet it is not uncommon to see people who, as they push farther back, the elbow joint is progressively relaxed and extended, taking away from the stretching of the triceps. This is where a help from a training partner can be worthwhile – all he or she has to do is ensure that the elbow remains totally flexed and push gently on it toward the back. It might be more comfortable to receive assistance if the partner sits on a bench.