example: say they underarm hook you and clinch. Instead of fighting them for that position, trying to get your arms under their armpits, Just elbow the head, repeadedly if necessarily, jab the eyes, slap the ears, bite, head butt, (step on their foot as you do these things for a little extra "nasty" in your defense and move forward) palm the chin and move forward and up, (the body follows the head).
example: if they pin both your arms down and clinch, you still have your legs and feet, headbutt. You can heel kick from here, step on the foot as you head butt forward, knee the groin, slap or palm strike the groin, inside the theighs are always tender (give a mean pinch with a twist! lol! Works on every big guy I ever did that to!

---------------------------------------------------------------------
This is where training with good BJJ guys really helps. You will never be able to headbutt a person who has underhooks on you - they tuck their head into your head and 'stick' with you. We do this all the time at Kamon
You won't be able to use your hands to attack (either in a strike or eye gouge etc) because you are sticking with them. Plus they will be initiating a takedown or hip toss or suplex from those kind of positions (or other nasty things), which means that you barely have time to even think about a headbutt.
Lastly, whatever, you do to them, they will do to you
Its a shame you are so far away. 10 minutes watching Sifu explain clinchwork would really shake up your wing chun
As for the link, I don't know it, but anyone who has access to youtube are free to post it here. Matsu??