Imam Khomeini through his theological works explains the matter as following:
Rather, the external senses are themselves the doors which shall be opened to the fire of Hell.
The fire of the hell of deeds and the physical hell encompasses the without and travels towards the within. Hence man is tortured from both the sides by these two hells: one emanates from within the heart and its flames enter the body through pia mater of the brain, and the other, which is the result of the vicious deeds, advances towards the inner being from without, and man is subjected to torments and pressures.
What sort of torment and torture it will be? God alone knows what pain and distress it will bring in addition to the burning and melting.
You imagine that the topological mode of the Hell’s encompassment is something that you know. Here things are surrounded only externally and outwardly; but in that world, encirclement will occur both externally and internally; it will cover the outer surface of the body as well as the inner depths of the human heart and being.