Trust the first impulse. That is the practical content of this changing line, and it is unusually direct: the movement in question is described as free of insincerity, and the advance following from it is called fortunate with no condition attached. Someone drawing it is not being asked to examine their motives so much as told that, on this occasion, the motives are already clean.
The difficulty in acting on such counsel is that it forbids the usual safeguard. Innocence in this hexagram is spoiled by calculation, so the ordinary move of pausing to work out what one stands to gain would introduce exactly the crookedness the Judgment warns against. The line asks for the thing done because it is right and obvious, at the moment it is obvious, and not for a version of it improved by strategy.
Correctly placed at the base of the lower trigram Zhen, this is the stroke from which the figure's whole movement rises, and a reading landing here usually marks a genuine beginning rather than a continuation. Nothing has yet been committed and nothing has yet gone wrong, so the querent is being told that whatever they are moved to do from a clear motive at this point can safely be done. What the line does not do is extend that assurance any distance ahead, which is why the positions above it have to take up the same question again under considerably harder conditions.