स तु दीर्घकालनैरन्तर्यसत्कारासेवितो दृढभूमिः
स: तु दीर्घ काल नैरन्तर्य सत्कारः आसेवितः दृढ+भूमिः
In order for the practitioner of yoga to be firmly established on the path of yoga, three conditions need to be fulfilled, as outlined in this sūtra:
a) it must be practised for a long time
b) it must be uninterrupted
c) the endeavour must be accompanied by devotion, in a spirit of reverence
Long, uninterrupted, alert practice is the firm foundation (for restraining the fluctuations).
While the sentence is perfectly valid, I don't agree with his translation because भूमिः which he is reading as foundation (that's fine) is a feminine noun. So it would have been सा instead of सः at the beginning of the sūtra.