The space between the tibia and the basitarsus contains a row of hairs - the pollen rake - and the bee uses this to scrape the pollen grains off the basitarsus into the gap. By straightening her leg the bee closes the pollen press and compresses the pollen which is squeezed up into the pollen basket on the outside of the tibia.
Here you can see the early stage of the formation of a pollen pellet in the pollen basket and see where the pollen has been squeezed up from below.
The bees on these Eryngium giganteum flowers were collecting nectar but not pollen. Their pollen baskets contained different colours collected elsewhere. This one has a small amount of grey pollen - possibly blackberry.
As pollen is collected the pollen pellet grows and is contained by the hairs around the pollen basket.
The tibia is concave to help retain the pollen.
As more pollen is collected the pollen pellet expands upwards. These photos also show that the hairs around the pollen basket become embedded in the pollen to help secure it.
The pollen remains firmly attached to the bee's legs until she gets back to the hive. She removes it with her middle legs into a cell in the comb where it is packed by the house bees. These last photos show the hairs around the pollen basket and the ridges of hairs on the inner surface of the basitarsus.