How does Samsung Pay work? Does it actually activate a magnet field that the payment card terminal reads? It seems like it doesn't use the chip but what are the details?
Samsung pay uses NFC or Magnetic Secure Transmission, whereas Android Pay uses NFC only and that's supposed to be the edge Samsung offers to users as also retailers who don't need to invest in NFC hardware
Magnetic Secure Transmission from Samsung website explains

Magnetic Secure Transmission (MST) is a technology that emits a magnetic signal that mimics the magnetic strip on a traditional payment card. MST sends a magnetic signal from your device to the payment terminal's card reader (to emulate swiping a physical card without having to upgrade the terminal’s software or hardware). MST technology is accepted at nearly all payment terminals with a card reader. ...
Your transaction and payment information will be kept private and secure with the use of tokenization. MST is more secure than using a traditional payment card and is as secure as paying with Near Field Communication (NFC).