The Silent Speaker - Forbes.com:

When we speak aloud, we're forcing air past the larynx and tongue, sculpting words using the articulator muscles in the mouth and jaw. But these muscles go into action regardless of whether air is sent past them. All you have to do is say the words to yourself and you're sending weak electrical currents from your brain to the speech muscles. Jorgensen's trick is to record those signals (known as electromyograms), process them with statistical algorithms and compare the output with prerecorded signal patterns of spoken words, phrases and commands. When there's a match, the unspoken turns into speech.