OK so I'm not a pilot so you'll have to forgive any glaring errors but my general interpretation is that....
In the cruise at high levels the a/c fly in Mach no's whilst at lower levels they refer to IAS. The instruction was for the a/c to fly an IAS of 290kts when they make the transition from Mach no to IAS. This varies but is usually in the mid 200 FL's.
(Takes cover before being shot down in flames for gross stupidity...)