How does wrapping plastic over credit cards help the machine read them?

Answer 1

It usually doesn't. If you slide it at a steady pace, not slow, but not super fast, then you can get most cards to read. I've tried the plastic bag trick. But I found that if I wiped the card's stripe on my left sleeve, I could get more of them to read.

The plastic bag will separate the magnetic stripe from the read head. This will decrease the signal strength picked up by the head.