Most likely in the past there was info passed down in ref; to how sanchin was developed ,but a good guess is better than a bad measure. So on my part it would be guesswork .I am a bricklayer/mason ,a basic principle of masonry is to understand expansion and contraction within a structure ,the structure is composed of various angles ,curves etc developing height in a structure may require a buttress ,sanchin dachi incorperates the principles of to;buttress the height of the structure,so the creator etc of sanchin must have had a knowledge of buttressing a structure under the dynamics of expansion and contraction utilzing angles ,curves etc within the kata .
So as a creator I need shapes angles, curves to build with ,also I know these shapes can fit into every type of activity ,with a circle i can create a segment ,or a chord ,or a ellipse I have a tight circle lengthening returning to a tight circle ,I know exactly were to to cut into it to shorten etc ,I can mold or blend .
I now devote all day every day working the shapes into a fighting structure ,I see angles curves shapes in all existing forms ,nothing much escapes my notice on this subject .I have the ability to defuse my attention to see more ,if I wish too it can be contracted to the minute . I can see the large and small in things ,this is why the mind infiltrates this structure ,this mind can't be approached ,but through long hard work ,there is no easy acssess to it
So a battle takes place to approach it ,that incoperates the very best of human endevour ,a great name for this creation must be Sanchin.