The proxy buffer high & lower are thresholds or trigger points in which certain actions (change in receive window) is taken by the F5.
Higher Threshold:
When the higher threshold is reached, the receive window that is advertised is no longer advanced i.e., whatever the receive window that was advertised last will remain the same and it won't be increased.
Lower Threshold:
Receive window will be opened up again and will be subjected to normal increase.
The increase or decrease in window size is shaped by normal TCP window sizes for data transfer.