Dopplers are small, battery-operated devices that send out high-frequency ultrasound waves. There's usually a handset, a built-in speaker and a transducer that's placed against your bump. The ultrasound waves pass through your skin and tissue and then bounce back, before being translated into sound. It's a similar process to being scanned in pregnancy but instead of pictures, it's sound.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/why-you-shouldnt-use-a-foetal-doppler-at-home-when-youre-pregnant