Motor reflex is reacting to stimulus without thinking, such as incepting a punch (which would also be combined with an attack). The more you practice, the faster your reflexes are. As coordination improves, you are able to use that to respond faster.
There are couple of ways of improving motor reflex: Repetitive and spontaneous.
With repetitive training one or many techniques are practiced over and over again in a stereotyped way. This type of training enables you to develop and refine techniques, but can prove to be ineffective in the real world, as repetitions are scenario based. Since no two situation are ever the same, scenario based training is limited.
In Wing Chun, training has a different emphasis, cultivating the use of energy through Fan Sau. This demands both a high degree of coordination and quickened motor reflex, so requires a more effective approach for improving the reflexes, which lies in the practice of unpredictable, unchoreographed partnership training. More random freestyle sparing are encouraged through the framework of Chi Sau, and are countered by the reflex action of techniques in muscle memory.
Chi Sau is a varied method of attack and defense training, providing fertile ground for the development of reflexes, to deal with what cannot be predicted. It is through this, that reflexes improve not only in quick delivery, but also responding to shifts in an opponent’s ‘energy’, regarding any subtle or not so subtle changes changes such as push, pull, raising an a arm, loss of contact, etc.
Relaxation is a prerequisite to good reactions, because tense muscles simply do not respond well, and they also make it very difficult to be sensitive to changes in the opponents energy.