Or by expulsion and retention of prāṇa
Expulsion is emission of the abdominal air through the two nostrils by a conscious effort; retention refers to the process of prāṇāyāma. By these two means one can attain steadiness of the mind.
The word Or means an alternative, so this is a means to steadiness other than meditations on friendliness, etc. The sense is that one should attain steadiness by some one of the means beginning with meditation on friendliness; a number of means are given, with the idea that one of them will be easier to a particular person and time and place.
Expulsion and retention separately or together. The first is emission of the abdominal air up to the limit through the nostrils, not by the mouth. Retention is the full process of prāṇāyāma, to the limit. Though prāṇa is to some extent restrained even by expulsion (alone), its function of going out has not been inhibited, and so retention is added, meaning prāṇāyāma.