As I understand it, one way is to just borrow again against similar stock. He borrows against stock bundle A for a while, and when that loan comes due, repays with a fresh loan against stock bundle B. A and B can be the same amount of stock, but as long as the line goes up, the loan against B more than repays the loan against stock A.

There’s intricacies and details in the process, but that’s how I understand the basic process goes.