Zapping citrus for 15 or 20 seconds helps break down carbohydrates in the fruit and encourages maximum liquidity...
Cold citrus doesn't release its juice as easily. This is due to two factors. First, the juice-containing vesicles in a citrus fruit are made of the complex carbohydrates pectin and cellulose. These carbohydrates are strong in colder temperatures and so are less likely to release the juice. Second, all citrus juices contain sugar, which means the viscosity of the juice also increases as temperature decreases. That's why it helps to toss the fruits into the microwave for 15 or 20 seconds just before juicing. You want to get the fruit just barely warm, not hot.