What does it mean to sustain a representation? It means to remain attentive to some specific fact or possibility, to refuse to allow myself to be diverted therefrom. If I have a boring project due at work, I must nonetheless continue to focus on it in order to get it done -- or, I could just let my attention drift and wind up writing on a blog.
Of course that is my example, not one of his.
This matter of sustaining representation (the key form of thought in this sentence, and in James' psychology of the will generally) plays into the dichotomy, also present in the above quotation, between the "impulsive" and the "obstructed." The impulsive fellow acts too quickly, plucking the tempting apple without wondering whose tree this is and what might be the consequences. There are various representations that he might be well advised to keep in his mind -- of arrest for trespassing, of an angry orchard-owner chasing him with a shotgun, of the extra pounds he may add to his waist by such impulsive eating. The obstructed fellow on the other hand can only see the reasons that block action, and cannot keep his attention on those that require it.
Thus, to think, by which we mean here to focus one's attention, to choose among the consequences of an act the key ones and to keep one's focus there, is the moral act. "The whole drama is a mental drama."
The apple orchard example is mine, too. Let's consider a couple of James at last. One involves an exhausted sailor on a ship, working the pumps to get the water safely off the ship and keep her afloat. The physiology of exhaustion weighs down upon the sailor and obstructs him in his work. What keeps him going? Thought! -- alertness, enforced perhaps by the image of the “hungry sea engulfing him.” Thus, thought overcomes the obstruction in his activity.
"The idea to be consented to must be kept from flickering and going out."