Imagine a goldsmith that is using tongs and a furnace to melt gold. If he constantly makes the fire too hot, the gold will get too hot. If he constantly sprays too much water on it, the gold will not be hot enough. If he constantly takes it out to examine it, it will never become refined. However, if he does all these things but each at their suitable time when needed, and he knows the nature of gold, he will have no problem at all in molding and refining it.
Just like that example, any practitioner needs to attend to these three qualities: focus, determination, and composure. If he properly attends to these things at the right time and circumstance, his mind will become pliant, brilliant, and pure, just like gold.