What it actually is
A protein chain carries charged groups. Away from the isoelectric point those charges repel each other, the structure is held open, and water sits in the space. At the isoelectric point the positive and negative charges cancel, the repulsion disappears, the structure collapses onto itself and the water is expelled.
Why it matters
Fresh meat sits uncomfortably close to the isoelectric point of its own protein, which is why it holds water so poorly without help. Moving the pH away from that point is the single largest lever available on water holding capacity, and it is most of what phosphate does.
What changes it
Anything that moves pH: alkaline salts upward, acidulants and fermentation downward. Salt shifts the apparent isoelectric point as well as increasing extraction, which is why salt and pH cannot be considered separately.
