I remember that ray mears episode, the method shown in that episode does work and produces huge amounts of sap fast
if you use that method be sure to understand that the trees fate is in your hands. Personalty I would only use that method in a survival situation as the risk of killing the tree by bleeding out and infection is an issue for me, the method above can produce 1/2 ltr in a few hours. When you have finished tapping knock the cuts close, take your time when tapping and be patient sap flows after a minute or so of a cut. After experimenting with various ways of tapping I use the following method: Make a stab then another a few millimeters lower than first one and leave your knife in this second stab, sap will not only flow from the one that you leave your knife in it will also flow from the cut just above.