Basically you can define it with 2, 3, or 4 points. Technically its always 4 points but you can link the start or the end points into the same point so when I call something a vector if the points are at the same location it becomes a vector with a length of 1 which is basically a point.
The morph always moves from the start vector to the end vector. With the various ways you can place the points you can create shapes like a line, a rectangle, any 4 sided polygon, a triangle with the start a point and the end a vector, a triangle with the start a vector and the end a point, an hourglass shape where the end vectors cross somewhere between the start and end, etc. You can have it run anywhere across the model so it can take up the whole model or just a small piece of it. So when you use several morphs on a model you want to set the blend mode to 1 Reveals 2 so that it lets the other morphs show through in the areas where pixels are black.
On the Start Tab you define the X/Y for two points 1a and 1b. If you check Link Points then it uses 1a for both points to make it easier to define a line instead of a region.
Same thing on the End tab. You are defining the vector where the effect will end.
You can do some pretty neat things where you reverse the points for the start or end vectors and cause the morph to look like it's twisting.
The length slider on the Start and End tabs defines the size of the head at the start and end of the morph so you can make it grow, shrink, stay the same, or drag it to zero to delete it.
The head duration on the options panel sets the amount of time the head takes to travel from start to end.
So if you have an effect length of 10 sec and drag the slider to 30% the head will take 3 seconds to travel from the defined start and end points and 7 seconds for the tail to make it across.
Colors: It uses the first 4 color palettes to define the head and tail colors.
Color 1: defines the starting color of the head
Color 2: defines the ending color of the head
Color 3: defines the starting color of the tail
Color 4: defines the ending color of the tail
If you want it to look like the tail is fading out then select Black for Color4.