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.