The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. Next follow the imprecations 1.
2. Let him know that breath 2 is the beam (on which the whole house of the body rests).
3. If any one (a Brâhmana or another man) should chide him, who by meditation has become that breath as beam 3, then, if he thinks himself strong, he says: 'I grasped the breath, the beam, well; thou dost not prevail against me who have grasped the breath as the beam.' Let him say to him: 'Breath, the beam, will forsake thee.'
4. But if he thinks himself not strong, let him say to him: 'Thou couldst not grasp him who wishes to grasp the breath as the beam. Breath, the beam, will forsake thee.'
5. And whatever the reciter shall say to one who speaks to him or does not speak to him, depend upon it, it will come to pass. But to a Brâhmana let him not say anything except what is auspicious. Only he may curse a Brâhmana in excessive wealth. Nay, not even in excessive wealth should he curse a Brâhmana, but he should say, 'I bow before Brâhmanas,'--thus says Sûravîra Mândûkeya.
252:1 The commentator explains anuvyâhâra, not as imprecations, but as referring to those who leach or use the imprecations, such imprecations being necessary to guard against the loss of the benefits accruing from the meditation and worship here described; such teachers say what follows.
252:2 Breath, the union of mind and speech, as explained before. This is the opinion of Sthavira Sâkalya, cf. III, 2, 1, 1.
252:3 If he should tell him that he did not meditate on breath properly.