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How to Blend Two Segments
see also: Continuity, How to Edit a Blend-Point

Segments can be blended in any way you like!  Here we will talk about the basics.

MotionDesigner lets you design any motion you like.  In other words, we make no assumptions as to what you want the motion design for.

Applications might include anything from computer animations to complex packaging machines.

I guess there are three uses of a motion design completed in MotionDesigner:

1. Motion Design intended for "inertia-less" systems, such as animations.
Here, the user is only concerned with ensuring that the sequence of positions replicate the animators intent. If she wants a step change in position so that the cartoon legs move instantly between positions, then so be it.
2. Motion Design intended for "inertial" systems, that is mechanical systems.
Here, the user is concerned with ensuring the machine performs well, and can satisfy all the motion conditions needed.  The motion conditions can be defined in position, velocity, acceleration and jerk.
3. Motion Design intended for nothing in particular!
Who knows the data is maybe intended for further numerical analysis not provided by MotionDesigner.

Let's assume case 2: Motion Design intended for "inertial" systems, that is mechanical systems.

It is assumed that the motion design will be downloaded to a servo motor or will be incorporated into the follower motion of a cam mechanism.  In these cases, you are be "asking" the cam or servo motor to replicate the motion you have just designed.

Position Blending
In Case 2, it is important that adjacent segments are continuous (that is "blended") in at least Position.
Mechanical systems do not like moving from one position to another position in no time! - it would be like a magic trick!. You would be asking the servo motor to be in one position, and "then" (that is, in no time at all) to be at another position.  The servo motor controller would get pretty upset at having to deal with such a large error in position.  At best, the servo controller would report back and error and an error handler would stop the machine in an orderly fashion. At worst, the machine would physically clash and something will break!  

In this case, you can use the "Match" icon in the Position Field of the Blend-Point Editor.  With the Match icon clicked, you can also specify the actual position value of the blend-point by using the "In-Specify" icon and the spin-box control.

Velocity Blending
It is also important that adjacent segments are continuous in Velocity. 
Again, mechanical systems do not like a sudden change in velocity in virtually no time.  It is called impact.  You might have experienced being hit from behind by another car travelling faster than you.  Something usually breaks, unless it has been designed for impact.

A request from a servo motor for a step change in velocity, requires an infinite acceleration.  This is physically unattainable. Again, the servo controller might be able to deal with velocity discontinuities, but it might not be pretty. 

In this case, you can use the "Match" icon in the Velocity Field Blend-Point Editor.  With the Match icon clicked, you can also specify the actual velocity value of the blend-point by using the "In-Specify" icon and the spin-box control.

Acceleration Blending
It is usually important that adjacent segments are continuous in Acceleration.  
Mechanical systems do not like suddenly changing their acceleration in zero time.  For example, if you support a 300mm ruler at one end over the edge of a bench, and drop a coin from zero height on to it (that is, the coin is a already touching the ruler and does NOT impact it) then you will see the ruler vibrate with a peak amplitude of about twice the final resting deflected position.  This is equivalent to a step change in acceleration.
You will also notice, that railway tracks and high speed roads do not have step changes in curvature (1/radius).  The trains would rock side to side as they passed over radius change if they existed..

With the Flexible Polynomial Segment Type, you can specify any value for acceleration and still use the Match Blend-Point icon.

In this case, you can use the "Match" icon in the Acceleration Field Blend-Point Editor.  With the Match icon clicked, you can also specify the actual Acceleration value of the blend-point by using the "In-Specify" icon and the spin-box control.

Jerk Blending
Finally, it is sometimes required that adjacent segment are continuous in Jerk.   
Continuity in Jerk results in the least mechanical vibration in the mechanical system following the motion design, at the expense of higher peak accelerations. If the duration of the segments can be increased, then the peak accelerations can be reduced to compensate. For a standard motion law, zero Jerk at the start of a segment will result in tiny amounts of displacement for the first 10 degrees of input.   Unfortunately, by specifying zero continuity in jerk to get a smoother motion,  the motion will generally have a higher acceleration.  This is essentially the compromise in standard law motion design.  How smooth can I make my motion before the peak velocity and acceleration become unacceptable?

With the Flexible Polynomial Segment, you can specify any value of jerk, and still use the Match Blend-Point icon.
In this case, you can use the "Match" icon in the Jerk Field Blend-Point Editor.  With the Match icon clicked, you can also specify the actual Jerk value of the blend-point by using the "In-Specify" icon and the spin-box control.