Crack velocity in rubbers, which in general monotonically increases with increasing applied strain, can exhibit a peculiar jump (discontinuous increase) at a critical strain. This phenomenon, which is called velocity jump or mode transition, has been known for decades and considered to be an important and relevant issue both scientifically and industrially. Nevertheless, until recently the mechanism of crack mode transition in rubbers had remained unclear. Our recent work of numerical simulation (Sci. Rep., 2017, 7, 42305) revealed that a non-monotonic temporal development of stress the near crack tip is produced due to viscoelastic nature of rubber-like materials, which causes the velocity jump. This interpretation of the phenomenon turned out to be consistent with findings based on a mathematically solvable model by Sakumichi and Okumura (Sci. Rep. 2017, 7, 8065), suggesting that the slow-fast transition is caused by change in behavior at the crack tip from rubbery to glassy nature. Consistency of the implication of these theoretical studies with experimental observations about the mechanical properties of rubbers is also discussed.