Hi, as i commented, i was not sure, so i asked in IRC... this was the answer:
[15:17] p7f: prea and postb are handles, indeed similar
to those for a besier curve
[15:17] the result curve will be tangential to the
pre_a - a line at
a, I assume
[15:18] best way to choose them is to try with some values first, see what result it produces, then proceeding from there
bojidarbg: so it should be also tangencial to
b - post_b?
[15:19] yeah, at b
[15:19] bojidarbg: thanks a lot!
Is not my merit this answer, but i post so others can see anyway.