In plain words
It is precisely by taking up such a black-antelope skin that the sacrificer, initiated into the rite, invokes the chandas.
The black-antelope skin, carrying white, black and reddish-brown colours, is the form of the three worlds; and so, wearing the black-antelope skin, the sacrificer becomes initiated into the three worlds. The initiated sacrificer is like a foetus in the womb. Just as the foetus in the womb, in whatever manner it is, remains covered by the mother's body with its fingers clasped into a fist, so too the sacrificer, in that same way, with a clasped fist, must remain covered by the body of the mother that is the chandas, in the form of the triple [Vedic] knowledge, wearing the black-antelope skin.
In the human view, the black-antelope skin is merely a hide. Because it is dear to the deities, it becomes, in sacrificial rites, [a source of] śarma [well-being]. For precisely that reason, the sacrificer, at the time of the sacrifice, addresses it: 'You are the very form of śarma. Give me śarma' — so he addresses it.
'Śarma' means happiness, bliss. The very form of bliss is Viṣṇu. Viṣṇu is the very form of the sacrifice. 'Yajño vai Viṣṇuḥ'. That Viṣṇu is the very form of the triple [Vedic] knowledge.