Why is my Dyson SV12 motor pulsing and cutting out?

If your machine detects a blockage, the motor will pulse and you won't be able to use your machine until the blockage has been cleared. If you try to use your machine when the motor is pulsing, it will automatically cut out. You should check the main body, wand, and cleaner head for blockages and clear them before reassembling. (Page 8)