When machine embroidering, one way to keep thread tension stable is to lay the spool in it’s side on your sewing table and let it unwind on it’s own. The weight of the spool may keep the thread taut. Another way is to put a baby sock or thread sock over the thread. This is especially helpful if you are using a cone.
What is a thread sock? It is a thin tube of nylon netting that is made for sergers to keep the thread from unwinding on its own. A thread sock is usually included on every cone of monofilament thread.