What is "Al Yish'u"?
Rashi: They should not think and speak [about vain matters]. This is unlike "v'El Kayin v'El Minchaso Lo Sha'ah", which means to turn to. 1
Hadar Zekenim: If it meant to turn, it would have said "l'Divrei Shaker."
Rashi writes that Al Yish'u is unlike Sha'ah regarding Kayin. In Bereishis (4:4), Rashi said that there it is like Al Yish'u!
Da'as Zekenim, Hadar Zekenim: Rashi there does not mean that it is like Al Yish'u here, rather, Al Yish'u El ha'Mizbechos in Yirmeyah or Yeshayah, which means to turn.
Why did Pharaoh command to make the work harder?