One of the more intimidating machines for people is the Walking Foot Industrial Sewing Machine, which can be found at pretty much any TechShop.
The Juki Walking Foot (at TechShop San Jose) has a very useful safety feature built in, called a safety clutch mechanism, which will automatically engage if there is ever a sewing problem (thread in the hook, etc.) to prevent damage to the machine. However, it can be quite expensive to call a professional out to re-engage the clutch if you don't know how to do it yourself.
In this Instructable, I will show you how simple it is to re-engage the clutch and how do it do safely.
First, for safety reasons, remove your needle, being sure to note which way your needle is facing and where the scarf sits in regards to the machine.
Once you have this verified, turn your machine off and discharge the power from the motor by pressing down on the machine foot until the motor completely stops.
You will need to press it down completely and hold it while turning the hand wheel towards you. Like other industrial sewing machines, you will probably have to press down slightly on the machine foot in order to turn the wheel.
Turn the wheel both towards you and away from you until you feel the pin engage and press in a little further than it had been before. Once you have the pin fully engaged, start turning the hand wheel towards you.
You will hear a few small "pop" noises as you turn the wheel (again try turnign it both towards you and away from you), but eventually you will hear one very loud and quite distinct "POP" which means you have successfully re-engaged the safety clutch. Once you have done this, you will see the bobbin spin while turning the hand wheel.