I've failed to grasp what my flute teacher was trying to get me to understand how to do vibrato... after a few lessons I nearly gave up! But I didn't... I search the Internet (what else a lifeless person could do)... and found this method taught by Nina Perlove:
She mentioned the technique was developed by William Kincaid long long time ago.
But I still can't get my vibrato sound natural... then I found Carl Petkoff's technique and realised I should whisper "Too Woo Woo Wooh" instead of the "too foo foo foo foo" that I've been doing!