From Aparoksanubhuti by Sri Shankaracharya

"The steps, in order, are described as follows: the control of the senses,

the control of the mind, renunciation, silence, space, time, 

posture, the restraining root (mulabandha), the equipoise of the body,

the firmness of vision, the control of vital forces, the withdrawal of the mind,

concentration, self-contemplation and complete absorption"

*these fifteen steps include the eight steps of Patanjali, but with a reorentation of meaning as will be evident from the following.

Four verse on yoga by Shankaracharya (Mula bandha)

Aparoksanubhuti

I bow to Sri Hari, the supreme bliss, the primary teacher,

the Isvara, the all-pervading cause of all the worlds.

One should know that a real Yoga asana is that
in which meditations flows unceasingly with openness and ease,
and is not that in which happiness is destroyed.

That which is the root of all beings,
that root which bonds all of the mind
(all thought and experience)
That is Mula bandha, which should always be served
(attended to) and which is fit  for rajayogins.

By making the vrtti (present pattern of mind the object of meditation)
still, and then intuiting it to be sacred (the infinite  Brahman), there is
the complete forgetting of the vrtti in the wisdom known as Samadhi.