मन्त्रवच्च यथान्यायं यज्ञोऽसौ सम्प्रवर्तते।|
एवं प्रवर्तिते यज्ञे विश्वामित्रस्य धीमतः।
आकाशे च महाञ्शब्दः प्रादुरासीद्भयानकः॥
mantravac ca yathā-nyāyaṁ yajño ’sau sampravartate
evaṁ pravartite yajñe viśvāmitrasya dhīmataḥ
ākāśe ca mahāñ śabdaḥ prādurāsīd bhayānakaḥ
mantravat ca = and recitation of appropriate mantras; yathā-nyāyam = according to the scriptural rules; yajñaḥ asau = the yajña; sampravartate = was conducted; evam pravartite yajñe = while the sacrifice was performed thus; viśvāmitrasya = Viśvāmitra; dhīmataḥ = by the intelligent; ākāśe ca = in the sky; mahān = a great; śabdaḥ = sound; prādurāsīt = there was; bhayānakaḥ = and frightening.
The yajña was conducted according to the scriptural rules and recitation of appropriate mantras. While the sacrifice was performed thus by the intelligent Viśvāmitra, there was a great and frightening sound in the sky.
Where is the possibility of rākṣasas arriving at a sacrifice where mantras that destroy rākṣasas are being recited? That is indeed impossible. The rākṣasas stayed far from the sacrifice and showered blood just as clouds in the sky [shower rain from a distance.] That is pointed out in the last line of this verse.
NOTE. Viśvāmitra’s fire sacrifice was conducted strictly according to śāstra. All activities we engage in should be in accordance with śāstra. This is stated by the Supreme Personality of Godhead thus:
tasmāc chāstraṁ pramāṇaṁ te kāryākārya-vyavasthitau
jñātvā śāstra-vidhānoktaṁ karma kartum ihārhasi
“One should therefore understand what is duty and what is not duty by the regulations of the scriptures. Knowing such rules and regulations, one should act so that he may gradually be elevated.” (Bhagavad-gītā 16.24)