At that time, he probably overlapped something he accidentally said wrong with another recording and the inserted recording was a lot louder than his voice throughout the rest of the video.
I believe it's more accurate to simply call it horizontal displacement. It is NOT the shear strain, nor can it be used to compute the shear strain. The actual stress-strain conditions within the test are very complicated and not well defined. It is not possible to develop a stress-strain curve for the soil from this test.
It is the strength that is left in the material (either soil or rock) after it's failure. It's because of the holding of the particles together by inherent friction of the material.
