What did Rachel mean when she called Bilhah's second son Naftali because "Naftulei Elokim Niftalti ... "?
Rashi and Rashbam: Based on the word 'Pasil', she meant that she was joined with her sister 1 to merit children.
Seforno: She meant that she was intimately joined with her husband with a Divine and holy union, because, together with her sister, they gave their maidservants to their husband in order to bear the tribes of Yisrael. 2
Targum Onkelos: She beseeched Hashem in prayer, and He answered her, and gave her a child, like her sister.
Da'as Zekenim: She suffered great pain.
Tosfos ha'Shalem (5): I strove to overcome her, and I succeeded.
Tosfos ha'Shalem (9): I was crooked with Simanim (ha'Chosemes and Pesilim were Simanim) with my sister [to help her fool Yakov], and succeeded. A Midrash says that when Yakov married Leah, Rachel was under the bed, and when Yakov called "Rachel", she answered.
Naftulei means covered, like Tzamid Pasil. There are Divine secrets covered from people why Leah gave birth and I did not.
Da'as Zekenim, Hadar Zekenim: She joined her regarding skilled descendants. Ohali'av from Dan helped Betzalel [from Yehudah] to build the Mishkan, and Chiram from Naftoli helped Shlomo [from Yehudah] to build Bayis Rishon.
Seforno: Leah must have given Zilpah to Yakov before the birth of Naftali (See Seforno).
And what did she mean when she added "Gam Yacholti"?
Seforno: She meant that, with the birth of Bilhah's children, she had achieved what she set out to do by giving Bilhah to Yakov.
Da'as Zekenim, Moshav Zekenim, Rosh: This implies that she exceeded Leah! One Chacham builder will come from Leah (Betzalel), and two from Bilhah - Ohali'av from Dan, and Chiram (who finished Bayis Rishon) from Naftoli.
Malbim: I was able to bear [b'Simchah that Leah gave birth and I did not, for I know that Hashem is Tzadik in all His ways].
Ha'Emek Davar: Also I can [be the cause of many sons, like Leah, for she ceased having children, and Bilhah can bear many].
Le'ah has four sons and she has not even one!How can she say "Niftalti im Achosi Gam Yacholti!"?
Oznayim la'Torah: Rachel figured that, since Le'ah had stopped giving birth, and Bilhah already had two sons, Bilhah would give birth to another two sons - four like Le'ah. Leaving her to produce the remaining four, 1 thereby giving her the edge over Le'ah.