for example:

1. Old house assessed at 200000 all paid off.

Equity line 100000.

New house 300000, need down payment 75000 from  equity.

You find a mortgage company  to pay 225000 for the difference to close on the new house.

Now you sell the old house.

Buyer and mortgage provider put up total of 200000 on the table, 75000 go to the bank that provides the equity line, and 125000 go to you.