Shakeing the finger up-the-neck/ down-the-neck direction minutely while applying pressure is the idea. Don't actually move your finger position but let the shaking originate or pivot from the wrist and elbow and essentually beyond. Its like shakeing yet keeping the instrument still and let it resonate toward your body(as far as how to think about it) but, in actuallity it's sounding through the string. Ask Buzzart about it. He plays a fretless bass. On the guitar, it's the same concept except, the shake needs to be up and down literally but, remember to let it resonate from your inner self/feel. Boy, that sounds deep but it's just a feel and you'll get it. I tryed some vibrato on my last lead at class and was a little stiff. It definately requires a feel and takes practice. Everybodys touch is unique and we are enjoying your playing Tigerpaws so shake the heck outta that string!!
Am I spelling "shakeing" right?