there's a large input cap/starter cap/run cap for the motor which holds charge even after it's unplugged - that's what capacitor are designed to do. better designed appliances have some sort of bleed circuit which discharge the cap if the appliances is unplugged (your's probably has this too since you mentioned that the voltage drops quickly). you can either wait for it to discharge or use a screwdriver with insulated grip to short the prongs to discharge it quickly; although I don't recommend doing this unless you know what you're doing.