Verse 7.4.1

संकल्पो वाव मनसो भूयान्यदा वै संकल्पयतेऽथ मनस्यत्यथ वाचमीरयति तामु नाम्नीरयति नाम्नि मन्त्रा एकं भवन्ति मन्त्रेषु कर्माणि ॥ ७.४.१ ॥

saṃkalpo vāva manaso bhūyānyadā vai saṃkalpayate'tha manasyatyatha vācamīrayati tāmu nāmnīrayati nāmni mantrā ekaṃ bhavanti mantreṣu karmāṇi || 7.4.1 ||

1. The will is certainly superior to the mind. When a person wills, he starts thinking. Then he directs the organ of speech, and finally he makes the organ of speech utter the name. All the mantras merge in the names and all the actions merge in the mantras.

Word-for-word explanation:

Saṅkalpaḥ vāva manasaḥ bhūyān, the will is indeed superior to the mind; yadā vai, when; saṅkalpayate, a person decides; atha, then; manasyati, he thinks; atha, then; vācam īrayati, he directs the organ of speech; tām u nāmni īrayati, he makes speech utter the name; nāmni, in the names; mantrāḥ, all the mantras; ekam bhavanti, are united; mantreṣu karmāṇi, the actions [are united] in the mantras.

Commentary:

The mind is very important, but it heeds to be guided by the will. Why? Because the mind is always wavering. It cannot decide. In the Bhagavad Gītā the mind is described as being vāyoḥ iva, like the wind. It is restless and difficult to control. Sometimes the mind is even compared to a mad elephant.

All of us have minds, no doubt, but not all have an equal degree of will power, or determination. For instance, anyone may utter some mantras, but unless they are recited with saṅkalpa, they do not mean anything. They are just words. When you add saṅkalpa to the mantras, then the words become active and powerful. This is why saṅkalpa is higher.