Sefer Od Yosef Chai explains this pasuk with an incredible story. He begins by saying that the strategy of the yeitzer hara is to trick a person and fool him into doing things until he has him trapped in his web. The way to fight back against the yeitzer hara is to be smarter than him and to trick him into leaving you alone. This is seen from the following story:
There was a man who was friends with the malach hamaves (in the way that the Gemara in Moed Katan describes how Shlomo Hamelech and several Amoraim were friends with the angel of death). One day, the man was speaking to his friend and made one request. He said, “You are my friend. I know that I can’t ask you to never kill me. I am aware that you are merely the messenger of Hashem and you cannot refuse to do your duty. I just want to ask you one favor. When you come to take me from this world, do not kill me before I have the chance to say Vidui. Don’t take me so suddenly that I don’t have the opportunity to say Vidui for my sins.”
The malach hamaves agreed and gave his promise. When the angel of death agreed to his request and gave his word, the man became very happy. He said, “Now I can live forever! I just won’t ever say Vidui and the malach hamaves won’t be able to take me because he promised not to kill me before I say Vidui!”
When it came time for this man to die, the angel of death saw that he had tricked him. He saw that the man made sure never to say the words of Vidui. (He never said Tachanun, which has Vidui in it.) He knew that he would have to find a way to kill the man. He would have to trick him, just like he had been tricked.
The malach hamaves disguised himself like a sick leper who was on the verge of death. He lay down in the middle of a street that this man walked down frequently. When the man passed by and saw someone laying prone in the middle of the street, he walked over to see if he was okay. He saw that the sick man was close to death and was crying weakly. He was shocked to see this and he asked the man, “Why are you crying?”
The sick man said, “I am a Jew. I know that I am about to die but I am an am haaretz and I don’t know how to say Vidui. How can I die without saying Vidui? Please can you say it with me? If you say the words, I’ll repeat them after you.”
He became full of compassion for the sick man. He was so moved that he forgot the rule he had for himself to never say the words of Vidui. He began to say Vidui with the sick man, word by word. He started with the first word “Ashamnu” and kept going, with the sick man repeating after him. When he got to the last words, “Ta’inu v’ti’atanu”, the sick man jumped up and grabbed him. He took off his disguise and revealed that he was, in fact, the malach hamaves.
The man trembled in shock and fear. He realized that his friend had fooled him. He asked the malach hamaves, “Why did you do this to me?”
The angel of death said to him, “You just said the words ‘ta’inu v’itatanu.’ This means: ‘I fooled others, so I was fooled by others.’ You tricked me by convincing me to swear not to kill you until you said Vidui, so I tricked you by pretending to be a sick man and getting you to say the words of Vidui.
“You have already said Vidui. It is time for you to die.”
And the malach hamaves took the man’s life.
We see from this story that even the malach hamaves can be fooled. So too, one must try to trick his yeitzer hara. When one is doing teshuva, he should try to fool his yeitzer hara so that it doesn’t attempt to find ways to stop him. Just like it always tries to trick us, we have to fool it.
This is what Dovid Hamelech did. When he got up in the morning, he would say, “Today is a nice day. I am going to the orchard to take a walk”, or “I’m going to rest today in my tower”, or “I think I’ll go to the bathhouse today to wash up.” He would say things like every day. But in his heart, he was planning on going straight to the bais medrash to learn with his students and to sit with the Sanhedrin and issue halachic rulings.
When he left his house and went out into the street, he would walk towards the bais medrash. When he got there, he would run inside and sit down to learn.
He only said that we going elsewhere in order to fool the yeitzer hara. If he would have said out loud that he was going to the bais medrash, the satan would have placed obstacles in his way to try to stop him. Therefore, he said he was going for a walk, but he went to the bais medrash and jumped inside to get away from the yeitzer hara. Regarding this, he said (Tehillim 119:58): “I considered my ways, and I returned my feet to Your testimonies.” He first acted as if he was going on the way to do something else, but then he turned his feet to go learn Torah.
We see that even a great tzadik like Dovid Hamelech has to do what he can to trick the yeitzer hara. Simple people like us certainly have to fool the yeitzer hara so that he’ll leave us alone. We will not be able to beat him unless we have a plan of how to trick him.
The word “Cheshbon” can also mean “tricks” (as in Koheles 7:29). Thus, the pasuk is saying that those who rule over their yeitzer hara “come to Cheshbon” – they come to battle the yeitzer hara by tricking him. Through these tricks they are able to “build the city of Sichon.” “The city” is a reference to a person’s body. Thus, the pasuk is saying that, in this way, we can bring our bodies to “sicha na’ah”, meaning to serve Hashem nicely and properly.