This is a passage from "The Mowing of a Field" by Hilaire Belloc. In the essay, he describes in detail the right way to make hay.
So great an art can only be learnt by continual practice; but this much is worth writing down, that, as in all good work, to know the thing with which you work is the core of the affair. Good verse is best written on good paper with an easy pen, not with a lump of coal on a whitewashed wall. The pen thinks for you; and so does the scythe mow for you if you treat it honorably and in a manner that makes it recognize its service. The manner is this. You must regard the scythe as a pendulum that swings, not as a knife that cuts. A good mower puts no more strength into his stroke than into his lifting. Again, stand up to your work. The bad mower, eager and full of pain, leans forward and tries to force the scythe through the grass. The good mower, serene and able, stands as nearly straight as the shape of the scythe will let him, and follows up every stroke closely, ...
The meaning of "stand up to (something/someone)" that I'm familiar with is to confront, to challenge, or to resist. But I don't know if that meaning fits well in the context.
And because I'm not sure about the meaning of the expression, I am also confused about the use of "again". I didn't think he said anything prior to this sentence that's relevant enough to emphasize that he's re-stating his point (by saying "Again,").