The following is a brief summary of a thought said over by R' Frand in his shiur this evening. I have attempted to reproduce this vort to the best of my ability. Any perceived inconsistency is the result of my efforts to transcribe the shiur and should not be attributed to R' Frand.
On Friday night we sing the Aishes Chayil poem which contains the phrase "Yadeha Shilcha Bakishur" which means "Her hands she stretches out to the distaff" (translation courtesy of Art Scroll). The Yalku Shimoni on Sefer Shoftim explains that this sentence in the Aishes Chayil is referring to Yael, the wife of Chever HaKeini who killed Sisra with a tent peg.
As described in Sefer Shoftim, when Sisra was leaving the battle field, he came to the tent of Yael. Yael recognized Sisra and gave him milk and cheese and he became tired. Once he fell asleep, Yael killed Sisra by pounding a tent peg into Sisra's head.
The Yalkut Shimoni then asks - why did Yael use a tent peg? Sisra came from the battlefield with his sword and battle accoutrement. Why did Yael not kill him with the sword? It would have been much safer for her to do it swiftly with the sword. What would have happened to her if Sisra woke up while she was pounding on the tent peg?
The Yalku Shimoni answers that Yael was observing the rule that a woman should not wear men's clothing (found in Ki Seitzei 22:5) which includes men's weapons. Rather than transgressing this commandment (although she certainly could have in order to save countless lives) Yael took the tent peg and for this she is praised in the Aishes Chayil.
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