Graeme,
I've been giving some thought to the precaution: "The tree needs to be chained/cabled or strapped in at least 2 places below the cuts at the suitable spacings, prior to cutting. Without this precaution the tree has a chance of tearing a slab out of the back, below the climber and departing with the head."
I wonder if this tearing could be minimized when using the back release, by the following. After setting up the hinge with a plunge cut, finish the backcut by starting from behind and cutting foward, thus leaving the holding "strap", closer to the center of the trunk.
Would this prevent the tree from tearing out a slab in the back? If it was done wrong I could see it causing a bigger chunk to tear off. What do you think?
Also, what do you mean by "free" grained?
Thanks and God Bless,
Daniel