Length is a measure of both Yin and Yang, including pathology related to Yin and Yang.  Yang puts the pulse where it is and Yin anchors it there.  The determination of whether a Long pulse refers primarily to Yin or Yang is made by analyzing other accompanying qualities in the pulses. 

Long pulses indicate either that the body is exceeding its limits or that you are feeling two organs communicating with each other.  A pulse might be Long because there is too much Blood or too much Yin in the form of Dampness.  Or there may be too much Qi focused on moving that medium, causing the pulse to spread.  If there is much too much Qi focused on a medium, the pulse can eventually scatter.  There could also be too much Wind causing the pulse to be Long.  A Long pulse can also indicate Exuberant Yang.  

A Short pulse indicates you either do not have enough Qi to allow the pulse to spread itself throughout its own position or not enough mediumship to fill that position.  A Short pulse indicates the Yin is being contained or restrained.

The extremely Short pulse indicates insufficient Qi, insufficient mediumship or both. A short pulse can also apply to the set of three positions as a whole, when the pulse is only detected in one position or two adjacent positions.