The Pali word Niivara.na: should be being translated 'Diversion', not 'obstruction' or 'obstacle, or check. The psychology is completely different when dealing with an obsticle or obstruction or check than it is when dealing with a diversion. An obstacle, etc. stops one's forward motion until the obstacle is eliminated; a diversion weakens but does not arrest completely the forward motion.
The simile in AN 5.51 is of a diversion, not of an obstruction:
The Buddha likens a person mastered by wishing for sense pleasure, hate, lazy ways and inertia, fear and trembling, and doubt and vacillation (the Nivaranas - diversions) to a stream which has been diverted and no longer has it's original strength or free will. Then he likens a person who has mastered desire for sense pleasures, hatred, lazy ways and inertia, fear and trembling and doubt and vacillation to a stream whose diversions have been closed off and which has regained its strength and free will.
Words are very important! You must see yourself as if in a stream. The hippies said "Go with the flow". The Buddhist says: "Escape the flow". The Buddhist does not say: "Try to stop the flow," or "Try to stop flowing within the flow."
SNP 5.1.56 See Bhk. Thanissaro's note in his translation. "Culaniddesa (Nd.II), the streams that 'flow every which way' are the streams of craving, views, conceit, defilement, corruption, and ignorance that flow out the six sense media."
SN 2.12.28: dhamma-sota.m: the Dhamma-stream
The Buddhist idea of the stream is two-sided. On the one hand it symbolizes the flow of the world; on the other hand it symbolizes the flow of one who has latched on to the idea of escaping the worldly stream. First one must recognize the worldly stream and it's dangers, then one can understand the value of making the effort to escape. That is the meaning of Stream-entry. The ability to see that everything, including oneself is in constant motion, flow, and that one has taken a portion of that flow and identified with it and that if that identification is not broken, that flow will take one on endlessly against one's will. The stream flows into the river, the river into the ocean the ocean evaporates into the clouds and falls back to earth again as the rain which flows down the mountain into the stream. It is constantly ending and reviving. Lust is a diversion from the task of escaping identification with the flow. If one considers lust, etc. to be an obstruction to escape, it conduces to despair because it is felt that there is no progress at all in other areas as long as one has any lust remaining at all. And this is not the case. Progress is gradual. It is a progressive diminution of lust, etc. Progress, escape, liberation, freedom is pleasurable, enjoyable. Something that is desired once it is seen in even a small way. When lust, etc. is seen to slow down that progress, to divert one's energy from effort to enlarge that enjoyment, it is seen as something to be got rid of for the sake of that higher enjoyment.