Under B1 and B2 returns two new, freshly created, bezier curves, such that if you concatenate them - they will create this curve. Proportion is something from (0; 1). B1 will be equal to Self for T in TBegin .. TMiddle, B2 will be equal to Self for T in TMiddle .. TEnd, where TMiddle = TBegin + Proportion * (TEnd - TBegin).