I am still trying to figure out where you think you get a gain here. The only thing I can think of is something akin to kite’ing checks. That the $50,000 will not be attributed to the HELOC until the following month, but will take effect on the mortgage at the immediate month. The problem is that as of recent banking laws, transfers take effect in 3 to 7 business days depending whether it is in-state or out of state (check and funds clearing laws). This means that the fund transfers (credit and drawdown on HELOC) will be attributed virtually simultaneously.
