Sahara Dry Racket Grip Spray coats the hand in a super-hydrophobic dust, so sweat is repelled away from the palm. You will still sweat, but the sweat will be removed before it is large enough to have any effect on friction, so the grip will stay strong. Sweat can migrate from the arms and back of the hand, so please spray the back of the hand too if you are sweaty.
You can also spray the grip, as this will help it repel the small molecules of sweat on the palms of the hand, keeping the grip dryer for longer. This will dry the grip slightly, with long term effects similar to those of grips getting wet from sweat. After the game or practice, wipe the grip with soapy water to remove the dust and increase the life of the grip.
Sahara Dry is safe as Silica is inert and aluminium does not corrode graphite. If used on the grip then you should wipe with soap and water after play. If you use overgrips then do the same.