Crush,
I see what you're saying regarding needing pressure to somehow get the grease to displace the air in the inner birf - where the balls and the star reside. Here's what happens when you pump grease into a knuckle low on grease. The grease goes into the knuckle in ribbons and spaghetti-like coils and simply falls to the bottom of the cavity - kinda like pumping grease into a room from a hole in the roof. Eventually, you fill to the point this stuff has piled up to the "roof" of the knuckle. When you drive, it gets heated and stirred around and the air works its way out of the churning grease as it heats and flows downward continuously. Park the truck and the room (previously empty) is 2/3 full now with an empty air space above and warm grease below. The ribbons of grease have all 'melted' and flowed into the bottom of the knuckle. If the birfield and star are below the "water line" of the grease, then they are completely submerged in fresh grease.
So, warm grease and gravity get the new grease down into the birfield joint itself. Somebody has posted the great diagram of the knuckle here that's in the FSM, which helps you understand it's one contiguous space in the knuckle from the birfield chamber all the way to the spindle tip - fed by the square plug. Frankly, I would not have believed this had I not done the experiment quite spontaneously before a repack, and as mentioned I believed fresh grease would not get to the birfields. I underestimated the amount of churning going on in the knuckle from the spinning bits. But I think if you visualize what's going on in there from my weak attempt above to explain it, it may be clear as mud (heh).
I think this highlights why it is so important to keep grease in there to the proper level (2/3 or more) because at the half way mark is the spindle bushing. A lot of folks here have trashed their spindle bushings and that gets very expensive.
DougM