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A giraffe's heart, which can weigh up to 10 kg (22 lb) and measure about 60 cm (2 ft) long, has to generate around double the normal blood pressure for an average large mammal in order to maintain blood flow to the brain against gravity. In the upper neck, a complex pressure-regulation system called the rete mirabile prevents excess blood flow to the brain when the giraffe lowers its head to drink. Conversely, the blood vessels in the lower legs are under great pressure (because of the weight of fluid pressing down on them). In other animals such pressure would force the blood out through the capillary walls; giraffes, however, have a very tight sheath of thick skin over their lower limbs which maintains high extravascular pressure in exactly the same way as a pilot's g-suit

That is the secret! It is also called a "blood sponge".

It is a unique (if I remember correctly the Okapi has a similar but only very rudimentary one) and very intricate system the giraffe developed. Basically it is an organ sitting just below the brain, with many blood vessel, which is packed closely together. As the blood pressure increases when the giraffe bends down, the increased pressure forces the vessels closer together. This hinders the flow of the blood by constricting the vessels. This blocks the higher pressure and limits the amount of blood coming through, protecting the brain, while at the same time ensuring that enough blood do reach the brain for it to continue functioning normally.