"In all directions simultaneously" means "without succession". You should feel the spatial vacuity in all directions and without succession. That is, you experience it instantaneously, not gradually, step by step. And the phrase "the Complete Vacuity comes about to him" means "he experiences Absolute Void all around". And this Absolute Void is really the unlimited space of Supreme Consciousness. There are two conditions to succeed in this practice: 1) yugapat (simultaneously). Remember: It is all at the same time, not stage after stage. You feel the void without succession. 2) nirvikalpamanāḥ (with a mind free from thoughts). To succeed in the technique your mind should be still enough before. A good method to achieve this is to concentrate on the breath. Every time you are sitting for meditation, begin your practice by concentrating on your breath. This concentration stills the mind. Afterward, you may do the aforesaid practice or any other one. You contemplate over a part or region of your own body as void now.
At the beginning, you practice this technique just for a little while. As you practice it more and more, you become liberated from thoughts and attain to the Śiva's state, which is free from any thoughts. The real "You" or Śiva is beyond mental processes. No thought can contain Him because He is the original source of mind. He is the root of all thought and not a mere mental object. He is indescribable because of His thought-free state. And He is You. When you realize this, you experience immense peace and bliss. It is that simple. The image of the world being completely burnt makes your mind thought-free.
The disappearance of the world is symbolic of the absence of knowable objects. When there is nothing to know, mind becomes introverted. And at that moment of absortion, you attain to the Śiva's state which is called here "the highest state of man". Even though it seems that a person attains to Śiva, you should not forget that he is always Śiva. In fact, everyone is Śiva.