Most slot machines in land-based casinos still use a sensor to determine how many coins or bills are being inserted. By interrupting these sensors with a pocket laser light you can make the machine think you're inserting coins or bills while you aren't.
Bankroll - Cash set aside for gambling. Professionals use the word to describe the money they use for 'work'. Your 'bankroll' increases when you win and - obviously - decreases if you lose!