This study investigates the microscopic mechanisms of pressure-induced amorphous-amorphous (AAT) transitions and crystallisation in silicon. Using a machine-learning potential and local structural analysis, the research reveals that the transition from low-density amorphous (LDA) to high-density amorphous (HDA) occurs through nucleation and growth, resulting in non-spherical interfaces, highlighting the mechanical nature of AAT. In contrast, the reverse transition occurs via spinodal decomposition. Further pressurisation transforms LDA into very-high-density amorphous (VHDA), with HDA as an intermediate state. The final amorphous states are inherently unstable and transition into crystals. The findings demonstrate that AAT and crystallisation are driven by joint thermodynamic and mechanical instabilities, assisted by preordering, occurring without diffusion. This unique mechanical and diffusion-less nature distinguishes AAT from liquid-liquid transitions. The study also identifies three amorphous forms (LDA, HDA, and VHDA) and two crystalline forms (β-Sn and sh) in silicon. Structural analysis shows that the LDA-HDA transition proceeds via nucleation and growth, while the HDA-VHDA transition occurs through a two-step process, consistent with Ostwald's step rule. The HDA-VHDA transition is driven by spinodal decomposition, unlike the LDA-VHDA transition. The study further reveals that the formation of β-Sn and sh crystals from amorphous silicon follows a two-step process, with β-Sn acting as an intermediate phase. Heating the HDA and VHDA samples at high pressures leads to crystallisation, with β-Sn being more stable at higher temperatures. The results highlight the importance of preordering in AAT and crystallisation, providing insights into diffusionless solid-state transformations. The study underscores the complex interplay between thermodynamic and mechanical factors in pressure-induced phase transitions in silicon.This study investigates the microscopic mechanisms of pressure-induced amorphous-amorphous (AAT) transitions and crystallisation in silicon. Using a machine-learning potential and local structural analysis, the research reveals that the transition from low-density amorphous (LDA) to high-density amorphous (HDA) occurs through nucleation and growth, resulting in non-spherical interfaces, highlighting the mechanical nature of AAT. In contrast, the reverse transition occurs via spinodal decomposition. Further pressurisation transforms LDA into very-high-density amorphous (VHDA), with HDA as an intermediate state. The final amorphous states are inherently unstable and transition into crystals. The findings demonstrate that AAT and crystallisation are driven by joint thermodynamic and mechanical instabilities, assisted by preordering, occurring without diffusion. This unique mechanical and diffusion-less nature distinguishes AAT from liquid-liquid transitions. The study also identifies three amorphous forms (LDA, HDA, and VHDA) and two crystalline forms (β-Sn and sh) in silicon. Structural analysis shows that the LDA-HDA transition proceeds via nucleation and growth, while the HDA-VHDA transition occurs through a two-step process, consistent with Ostwald's step rule. The HDA-VHDA transition is driven by spinodal decomposition, unlike the LDA-VHDA transition. The study further reveals that the formation of β-Sn and sh crystals from amorphous silicon follows a two-step process, with β-Sn acting as an intermediate phase. Heating the HDA and VHDA samples at high pressures leads to crystallisation, with β-Sn being more stable at higher temperatures. The results highlight the importance of preordering in AAT and crystallisation, providing insights into diffusionless solid-state transformations. The study underscores the complex interplay between thermodynamic and mechanical factors in pressure-induced phase transitions in silicon.