Krishna now ties "Sat" directly to spiritual practice. Steadfastness in sacrifice, austerity, and charity — the unwavering commitment to keep at it — is called Sat. And any action performed for the sake of Brahman is also called Sat.
Two things earn the name "Sat" here. First: staying the course. Not abandoning yajna when it gets inconvenient, not dropping austerity when it gets hard, not stopping charity when the mood passes. Persistence in good work is itself a form of truth. Second: dedicating the action to Brahman rather than to one's own benefit.
A lamp that burns steadily through the night, without flickering, without giving up — that steadiness is what "Sat" looks like in practice. It is the quiet constancy that turns an ordinary life into something meaningful.