When did He command Shechakim?
Radak: He already commanded it to give to them manna.
When was "v'Dalsei Shamayim Pasach"?
Radak: When He gave to them manna. This is a metaphor for the abundance of manna that He made fall for them, like "va'Arubos ha'Shamayim Niftachu" (Bereishis 7:11).
Why does it mention Shechakim and Shamayim?
Radak: They should have realized that just like He can command them to give manna [He can command them to give meat].
Malbim: Shechakim is higher; it refers to a miraculous influence above the natural influence that comes from Shamayim. Manna did not come from Shamayim, rather, from Shechakim to Shamayim; through the open doors of Shamayim, it fell to the land.
