{112}<111>-type twin is a common twinning structure in quenched carbon steel. As carbon content increases, the density of the twin becomes high in the quenched state. Researchers have suggested that understanding such twinning mechanism may help us to understand the martensitic transformation in steel. {112}<111>-type twin is also commonly observed in other body centered cubic (bcc) metals and alloys, especially deformed under the conditions of low temperatures and/or high strain rates. Yet, due to the intrinsic non-close-packed structure and the rapid speed of twinning process, the mechanisms of twinning nucleation, growth and termination have not been clearly understood although phenomenological mechanisms such as the classical shearing mechanism, dislocation mechanism, or shuffling mechanism, etc., were proposed. Recently, after reviewing numerous investigations on {112}<111>-type twinning process both experimentally and theoretically in bcc metals and alloys, it was found that the twinning boundaries are always embedded with ω phase, i.e., the displacement of the first layer of the twin is 1/12 <111> for ω instead of 1/6 <111> for twin, thus, an ω phase-related {112}<111>-type twinning mechanism (so-called ω lattice mechanism) in our previous study is proposed. In order to better understand the ω lattice mechanism, in this work, a detailed description of the whole process of nucleation, growth and termination of the {112}<111>-type twinning was offered by using the atomic lattice model. The model shows that the twin could nucleate during ω→bcc transition process, and then grow up by extending or merging of twin embryos, and finally terminate during encountering the different ω variants. Such two-dimensional atomic model can be extended to three-dimensional one, which can finally explain the formation mechanism of an internal twin in one bcc crystal. Moreover, the model suggests that the diffuse ω lattice (ω_diff) between the ideal ω lattice and bcc lattice (in the twin boundary) plays an important role in promoting the transition of ω↔bcc during twinning nucleation and growth processes. The results suggest that the {112}<111>-type twins are phase transition twin or phase transformation product.