Overreaching and...

Overreaching is a definition for the short-term increase in training
stress .Is necessary Overreaching(not Overtraining)to stimulate change/improvening in sprint,hypertrophy…?Why?

Bump!Bump!!!

This is based on the supercompensation model of the training effect. Essentially, you apply a stimulus that is more than the organism is used to handling. After a brief period of regression, the body responds by overcompensating so as to be able to handle that training stress more easily next time. By properly timing these stresses to coincide with the supercompensation, one can build off of the peaks and see increased performance.

This is based on the supercompensation model of the training effect…but is better more Volume or more Intensity(for overreaching)?

I like to think of the overall effect as the integral of the curve - that is, the total area under the curve. So you can think of volume as being width and intensity as being the height. You can focus on either one depending on your specific goals but generally I think that intensity is more important for the short sprints. In the weight room, volume is probably more applicable for hypertrophy.