Seifly,
I will try to explain with my poor English:
Contrary to what is known, suspended water in the air does not turn to form of ice at 0C or below. It needs a surface to change into ice format. In another word, a water droplet at -20C is still in liquid form until it hits to aircraft surface, sticks and freezes: Icing.
However around -40C, the water does not need to find a surface to freeze, it freezes on its own. Then, when it hits to an airplane it cannot stick but bounces away: No icing
While climbing, once you see -40C, no more TAI is needed becuse as you climb OAT will be even colder. While cruising, as long as OAT -40C or below no TAI is needed too.
While you are descending the things are different, even the OAT is -45C now (no icing possible), it will be warmer than -40 C soon as you keep descending (icing is anticipated). So TAI is needed now, as you are starting to descent even OAT is colder then -40C
Regards