When I took a class from a cheesemaker who made tomme-style and washed rind cheeses, she said that pressing under the whey was one of the tricks she used to get consistent curd knit and texture.
All she did was let the curds settle to the bottom, rest the curds for 5-15 mins (depending on cheese type and pH), and then pour off most of the whey until the curd mass on the bottom appeared. Then, she took her hand and pressed into the curd, expelling more whey and knitting the curd mass together more. She pressed maybe a total of 20 times, rather gently but with constant pressure... for about two minutes, 5-10 secs per press. Then the resulting curd mass came out all at once into the cloth-lined mold, where it was pressed some more under very light weight (10 lbs). The curd mass was not broken up or milled between pressing in the pot and pressing in the mold. Cheese wheel was brined after about 12 hours and being flipped in the mold every 2-3 hours.
BTW, these were award-winning cheeses.
Hope this helps! My guess is that pressing under the whey helps to get rid of air space and knits the curds early on, so that the CO2 has no escape paths along micro fissures and must form large bubbles.