The initial degradation during negative bias stress is often assumed to be due to hole trapping, while the generation of interface states may dominate at longer stress times. We conduct a thorough study of short-time negative bias temperature stress and relaxation using ultra-fast measurement techniques in the micro-seconds regime to clarify the physical mechanisms behind the responsible hole trapping phenomenon. We observe that the extracted degradation of the drain-current DeltaID or the threshold-voltage DeltaVTH can be well fitted by a logarithmic time dependence. Only for stronger stresses, that is, higher temperatures and/or voltages, the data shows a detectable deviation from the logarithmic behavior, allowing for a power-law fit. The exponent of this power-law is about 0.04 and thus considerably smaller than the typically reported long-term exponents of about 0.12 to 0.15. We finally observe a strong field- and temperature-dependence of the initial degradation, which is incompatible with the frequently assumed elastic hole trapping mechanism but favors a thermally activated hole trapping process.