I think you are referring to a contoured shank which is what the manufacturers do to create space between and ankle and the shank. The art is in the execution and many get it very wrong, which puts the foot too far away from the tree. Bashlin aluminums (BD14B) have a very gentle contour, and are as good as it gets when it comes to keeping the foot close to the tree..
Offset refers to a stirrup which has been pushed forward, which puts the gaff in a more optimal position. When it is done correctly the stirrup is also angled so it sits flat on the climbers foot, which more evenly distributes the load.
Next is whether the gaff is straight with the shank, or is offset (a slight angle towards the toes), which creates a much better and forgiving ride. I believe Bashlin was the first to do this and most of today best designs have followed (for good reason).
Last is the gaff itself, and what most get horribly wrong..
Best tree gaff in the biz-
View attachment 92455
One of the worst-
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I believe the only 2 manufacturers that are making spurs that tick all these important boxes are Bashlin and Backingham..
Buck just released a new set which looks very promising. Lightweight. Non contour design. Offset and angled stirrup. Offset gaff with a nice profile. Will be spending some time in them when the rains let up. Will report back.
https://www.wesspur.com/SPU811-ArborAir-Tree-Climber-