I understand that they have an accelerometer, but I am a bit unclear as to whether the devices like the Vivosmart4 that my wife uses, or the Garmin Instinct that I use actually measures. I have read that when climbing stairs that one should not hold onto the handrail because the device may not accurately measure the steps, so that tells me that the device needs to see some arm movement, (I think) but does it also sense the sort of "bumping" movement that one's body makes when one walks? If I walk with my hands in my pockets will the device not sense my walking properly?
It reads a walk based on forward + up/down motion. You still get both movements with hands in your pocket, but you lack the up/down portion if your hands are on a handrail or shopping cart handle.
