Complex sentence: Instead, the metal necks and work hardens, increasing the tensile strength to very high levels, sometimes as high as 2000 MPa, causing the adjacent material to neck down, and this continues until the entire piece is much longer and thinner.
Split into simpler sentences: Instead, the metal necks and work hardens, increasing the tensile strength to very high levels, sometimes as high as 2000 MPa. This causes the adjacent material to neck down, harden, and this continues until the entire piece is much longer and thinner.