It could also be said - when your speed reaches 290 maintain it.
When descending at a constant mach number, IAS increases. if descending at m.78, depending on the day, at some stage the IAS will reach 290. At this point, the pilot will maintain 290.
The "transition" in the descent will depend on what the chosen (or imposed) descent indicated airspeed is. i.e. imaging descending at m.xx which was giving you an indicated of 200kt in the cruise. Initially during the descent the pilot maintains M.xx and therefore the IAS increases.
If ATC have said "on transition maintain 210Kt" the changeover will happen early. If on the otherhand they want 320Kt then the transition will happen much later.
There are two limiting speeds for jet aircraft (and a few turboprops) indicated airspeed and Mach Number.
At the lower levels, the indicated airspeed limit is reached before the limiting mach but at some level that limiting indicated airspeed will equal the limiting mach number. Going higher, the limiting mach number will require you to fly at a lower IAS i.e. it is impossible to exceed the limiting IAS without exceding the limiting mach first.
Thus in the climb, the aircraft is climbed at a chosen IAS until the chosen mach number is reached. Depending on how high the climb IAS the chosen mach number may be reached earlier or later.
In the descent it is the opposite.
Please note that IAS = indicated airspeed. TAS (Actual speed through the air and printed on your strips is different).
regards,
DFC