Practice is the effort at steadiness in it
Steadiness is the tranquil flow of the mind without mental processes. Practice is the effort thereto, the vigour, the enthusiasm, in undertaking the discipline to that end.
To explain the practice for it, he says, Practice is the effort at steadiness in it. in it means in their inhibition. The word steadiness is in the locative case, to show causality. The steadiness which is the cause of the inhibition of the mind is the result of effort, and the effort which is its cause is practice, the tranquil flow as it were of a stream free from mud is a transformation into a pure form of a mind without mental processes, which have been inhibited.
Effort: vigour, enthusiasm, are synonyms.
Practice is undertaking the discipline, the yoga discipline of restraints and observances and the others listed in sūtra II.29, to that end.