The ten essentials of Tai chi (太极拳十要) were taught by Yang style master the late Yang Cheng Fu （楊澄甫）. He was the most famous teacher of Yang style tai-chi form. His ten essentials are the best classic guidelines applicable to all styles of tai-chi forms.
Tai-Chi essential 1 of 10: 虚灵顶劲 (Head with chi)
虚灵顶劲: 顶劲者，头容正直，神贯于顶也。不可用力，用力则项强，气血 不能流通，须有虚灵自然之意。非有虚灵顶劲，则精神不能提起也。
My rendition of essential 1: “During practice: act as if an object is being balanced on top of one’s head”. Since the imaginary object has to be balanced and not to fall off, the force needs to be soft, operated from bottom, and with the mind being in an alert state.
Paul’s additional comment: for best results, this essential act must be performed together with another important act* called 收尾閭 （tilting up one’s coccyx）. Tilting up the coccyx is done through slightly contracting one’s perineum). Through contraction and mind-focus, the perineum and its surrounding muscles become the centre force to “balance the (imaginary) object on the head”. It acts through the spinal cord which forms as a connecting rod.
Through the combined effects of these two actions (虚灵顶劲 and收尾閭), the spinal cord defines the centre-line of the body which is essential in doing the tai-chi form correctly.
* Not that this is less important than the essentials, just for the purpose of clearer exposition.