It's pretty difficult to get every time, but it's worth a shot for double credits.

Basically, you need to exit (press 'B') and re-enter (press 'A') at the right moment.

The moment occurs immediately after:
"Firewalls Removed, full access granted"
sign comes up. Exit and re-enter quick enough, and you can hack the terminal or data pad once more.

You get twice the credits.

Some people may find it easier to press B, A, B, A
But doing that there is a chance you will end up exiting permanently.
Experiment to see which works well for you.