Bezier controller keys

I'm trying to figure out how to calculate the in and out tangents

I can edit them but I dont know whats the math or the logic behind it... if anyone knows a topic about it I`d would really appreciate that... I can`t find anything so far.

basically k.inTangent = [0.2, 0.02, 0.112] will edit the xyz tangent... quite easy so far but what i'd like to know is what really 0.2 represent and how it is affecting the tangent.