  A recently discovered modified low-temperature baking leads to reduced
surface losses and an increase of the accelerating gradient of superconducting
TESLA shape cavities. We will show that the dynamics of vacancy-hydrogen
complexes at low-temperature baking lead to a suppression of lossy nanohydrides
at 2\,K and thus a significant enhancement of accelerator performance.
Utilizing Doppler broadening Positron Annihilation Spectroscopy, Positron
Annihilation Lifetime Spectroscopy and instrumented nanoindentation, samples
made from European XFEL niobium sheets were investigated. We studied the
evolution of vacancies in bulk samples and in the sub-surface region and their
interaction with hydrogen at different temperature levels during {\it in-situ}
and {\it ex-situ} annealing.
