What is "Beis Rimon"?
Rashi: [Rimon] is the name of idolatry [that Melech Aram served].
Why will Na'aman bow?
Rashi: He will be forced, for his master is bowing while leaning on him.
Why did Na'aman ask Hashem twice in this verse to pardon his bowing?
Malbim: He will be forced to bow in two ways. (a) When his master is bowing while leaning on him, Na'aman must bow. According to letter of the law it is permitted, since Na'aman does not bend in order to bow, rather, in order to enable the king to bow. It is forbidden only due to Maris ha'Ayin (lest it seem that he transgresses), like we say that if a thorn became lodged [in one's foot when he is] in front of idolatry, he may not bend down to remove it (Avodah Zarah 12a). Surely Hashem will forgive this! (b) Afterwards, Na'aman will need to bow by himself, lest the king sense that he denies idolatry. For this, he asked pardon "ba'Davar ha'Zeh" - that I will take earth to build a Mizbe'ach and designate my Avodah only for Hashem. Through this, Hashem should pardon the latter bowing, since He knows that I designated my Avodah only for Him. I bow in Beis Rimon only due to fear of the king. A Nochri is not commanded about Kidush Hashem (Sanhedrin 74b). 1
So Tosfos (75a) explains the Gemara according to some texts; if he were commanded, Elisha should have explained that he is permitted only in private, but not in public. According to Rashi, and our text, perhaps he is permitted only in private, but in public, he is commanded about Kidush Hashem!
What is "b'Hishtachavayasi"?
Radak: This is a noun (a gerund, i.e. in my bowing). This is like 'there were 13 Hishtachavayos in the Mikdash' (Shekalim 15b).
