We study the implant-induced hydrogenated defects responsible for the Smart Cut™ layer transfer of Si (001) films. Different experimental methods are used to quantify the time dependence of the defect evolution and interactions during isothermal annealings. An optical characterization technique was developed for the statistical analysis of the formation and growth of micrometer size microcracks in the buried implanted layer. We show that the formation of molecular hydrogen is dominated by a transient phenomenon related to the rapid dissociation of the hydrogenated point defects. The impact of the ${\mathrm{H}}_2$ formation kinetics on the microcrack evolution is described and the physical mechanisms involved in their growth are identified. A comprehensive picture of the fracture phenomenon in H implanted Si leading to the full layer transfer is proposed and discussed.