Extension is most easily accomplished when the rock is under no confining pressure or low confining pressure. When rocks are under immense confining pressure there is no room for an extension in any direction. Hence, we say that the rock is completely supported. When a rock has no place to extend it becomes very strong and is more difficult to deform. When you pulled very hard on the putty, it snapped into two distinct pieces, whereas when you pulled slowly on the putty it just stretched and showed no sign of breakage.
Earth materials flow by a number of mechanisms.
All crystalline materials have defects in their lattices called dislocations. These are places where atoms are missing. Crystals have millions of these holes. Under stress these holes in the atomic lattice of a crystal can move by having atoms jump into the holes. Once an atom jumps into a hole another hole is created where the jumping atom previously was sitting within the lattice.