I find my really high quality steel tools (screwdrivers) magnetise well naturally. They get knocked around..
Its all about creating random directions throughout the material, thereby resulting in no net direction of magnetism. I GUESS that the powered ones are AC powered coils, rotating the magnetic fields in opposite direction, hopefully not creating a particular direction??! thats my guess..
Fixed ones are only good at demagnetising the magnetic field of the opposite direction. So its technically difficult to de-magnetise.
The only things ive demagnetised are Neo magnets, by heating them above 80 deg C. :((((
I've had one of those Magnetizer / Demagnetizer for years and it works great as a magnetizer but not so much as a demagnetizer. Maybe the newer ones work better.
Not only can it demagnetize screwdrivers pretty quickly (magnetized with one of the silly magnetizer/demagnetizer boxes) but it can also magnetize them slightly.
To magentize - place the screwdriver on the "operation field" (Chinese for the device part the covers the magnet) and press the button for a few seconds. Leave the button and only then remove the screwdriver.
To demagnetize - place the screwdriver on the "operation field" and press the button for a few seconds. While the button is pressed pull the screwdriver slowly away from the "operation field" and only when at least 10cm from the device leave the button.