Often called "velocity banking."

There comes a point where they are depreciated for tax purposes and the velocity of money is accelerated if you chose to switch out the portfolio to different real estate investments.

It's called the velocity of money. 

I mostly flip, and operate at a high velocity.

We are not too worried for traditional financing but would like to obtain a LOC to increase our cash velocity for purchasing rental properties. 

Velocity banking is a phenomenal financial strategy that has allowed me to grow my wealth much faster than I otherwise would have.

As long as you have 20% equity in your primary, the HELOC would give you more spending power, and in the meantime you could use it to attack your principle owed on the current house, reducing your DTI.Check out the WiseGuysInTies video on velocity banking if you want a crash course in using lines of credit to pay down a mortgage fast.

Look online for Velocity Banking if you want more info.

You will increase velocity with each property.

The Kwak Brothers also have some very decent videos on Velocity Banking.