During blood flow restriction training, lactate accumulation causes water to be drawn into the muscle, leading to swelling.

“the reason it does that is that lactate accumulates and that changes the osmolarity within the locally so that and that's dangerous so your body is not going to let some toxic process harm you so it in order to compensate for that it draws in water to equalize the osmolarity and that water causes your muscle to swell you really are swollen”