The Freehand is considered a narrow bodied mei tai (most vary in width from 12-20" or so, Freehand is not the most narrow but more narrow then some) so the triangle allows the bottom to kind of work like a wider body mei tai (in how it holds the legs). It just supports a bit more of the length of the leg, then would another mei tai of the same more narrow body width, but without the triangle. It is a really cool feature!!
However, I wanted to add that nearly all mei tais, even those with the more narrow 12" wide body, support a child through the legs and back in a sitting position, they are all good for that!! Even 12" is not narrow enough to leave the baby to dangle. The Freehand Onbuhimo is quite narrow and does not have the triangles, and it is still very supportive.
Whether one likes a wider or more narrow body is personal preference (for the wearer and the baby).