It is extremely hard to keep the mind occupied with just one idea. The same difficulties were also ran into by the early sages. Arjuna, in Gita had mentioned that commanding the head is an impossible matter on earth. Thus, Krishna proposed him that though mind control is not easy, it can be made steady and quiet by standard practice of abhyasa and vairagya. Nevertheless, he’s warned that yoga is very hard for individuals whose thoughts aren’t managed and steady. Pantanjali, in Yoga Sutra has highlighted these two qualities to attain mind control.
Head is like a displaced pond with several impurities. You must stop the inflow of fresh impurities and after that remove the impurities that are present to clean the head. Abhyasa is one for purifying the head of the practices. This is a period after practicing attention for some time that an individual attains. At the start of dhyana, the head is more steady and just an individual idea about an item appears in the mind. Now it’s safe to say the state of dhyana is achieved.
The second one is not entirely physical. It indicates entire concentration of thoughts into itself. Here, the head isn’t connected with any outside thing. It becomes utterly in this state. This head is assumed to stay quiet, still and sensitive such that any past, current and future occasion that may have occurred everywhere in the universe can be understood by it.