SYLVAIN is explaining that the best choice for grafting is a leaf node that has not yet sprouted.
Each such leaf node contains an embryonic sprout that will start to grow after grafting.
If you choose a node that has already sprouted (what Sylvain calls 'a fork') growth will often be very slow because new shoots do not easily develop at this point - although they probably will eventually.
In the finger lime twig photo below I have marked a fork - which is the bad choice - and a leaf node, which is a better choice for grafting.
In Laaz's diagram you could extend the red box to the bottom left, so that the fork in the twigs is also included as a blind bud.
Hope this is now clearer!
(Click photo to enlarge.)