The block salt sits in 10 mm of water. Before a regeneration starts, that water is saturated with salt. After the regeneration the brine is replaced with fresh water which then dissolves more salt from sides at the bottom of the block.
The brine is heavier than water, so as the fresh water touches the side of the salt, it dissolves and forms brine, which sinks to the bottom. The fresh supply of unsaturated water is therefore at the top and the process works in the form of layers.
It slowly dissolves into the centre until eventually you have a tiny ‘stiletto heel’ of salt right at the bottom centre of the block holding the block up. Eventually this physically collapses and the salt drops, and the process starts all over again.
Typically, it is taking a 10 mm bite into the block at each regeneration.