For the colony a shade break allows the bees to concentrate on tasks other then foraging for water to keep the hive temperatures down. Hot hive temperatures put stress on the bees, it makes the combs unstable and causes the bees to beard on the outside of the hive. Bearding is when the bees in the hive get too hot and they begin festooning on the front of the hive. Also, hot hive temperatures make it very difficult to work the hive because the combs become soft and fragile and are prone to collapsing from the bar. Bees can adapt to very high ambient temperatures but problems arise from penetrating radiant heat from the sun or when the air is still and there is no breeze to push the hot air away from the hive.
For millions of years bees have chosen cavities in live deciduous trees to inhabit which shade the colony with their leaves and branches during the hot hours of the day. Modern hives are often placed in full sun, which the bees may be able to cope with but it is not ideal and there are costs to their health and productivity.
During high summer temperatures the bees have set up the perfect swamp cooler, in-fact it is a very sophisticated cooling system, consisting of deliberately curved combs that are constantly coated with a layer of water which the bees fan with their wings, creating the cooling effect. Groups of bees form complex arrangements of fanners in rows to optimize the air flow across the water coated combs. When the bees tire, they switch off and let other bees take over their fanner posts so that they can rest and recharge. Imagine that your swamp cooler or air conditioner is keeping your house nice and cool and then someone opens a few windows and the sliding glass door! Your cool house won't be all that cool now.
It is the same when we take out the falseback in a top bar hive. Suddenly the bee’s air conditioner is less effective. Listen to the sound of the hive moments after you remove the falseback, the fanning hum of the hive will greatly increase. So when working your hive in the summer, only work with one or two open bar spaces and frequently return the falseback into place until the bee's sound gets lower. If you encounter newly drawn out, pure, white comb while working the hive be extra careful as the combs can easily drop in the hive because the swamp cooler now has an 'open window or door'. One of the most common causes of cross combing is heat related and is actually not even a cross combing mistake by the bees, it is intentional. To counter the direct heat from the sun, the bees start building curved combs which make it easier for the bees to circulate the air flow in the hive keeping the temperature down.