High piezoelectric activity of many ferroelectrics has been the focus of numerous recent studies. The structural origin of this activity remains poorly understood due to a lack of appropriate experimental techniques and mixing of different mechanisms related to ferroelectricity and ferroelasticity. Our work reports on the study of a uniaxial Sr 0.5 Ba 0.5 Nb 2 O 6 ferroelectric where the formation of regions with different spontaneous strains is ruled out for the symmetry reason and where the interrelation between piezoelectricity and ferroelectricity can be inspected in the isolated fashion. We performed X-ray diffraction experiment on a single crystalline sample under alternating electric field and observed unknown hidden-in-the-bulk mechanism, which suggests that the highest piezoelectric activity is realized in the volumes where nucleation of small ferroelectric domains takes place. This new mechanism creates a novel roadmap for designing materials with enhanced piezoelectric properties.Electromechanical coupling is the ability of some solids to convert mechanical energy into electrical and vice versa. Solids exhibiting linear electromechanical coupling are called piezoelectrics: they may become electrically polarized under a mechanical stress or mechanically deformed under an electric field. Piezoelectricity closely co-exists with ferroelectricity -the ability to switch spontaneous polarization states under an electric field. This letter reports on the observation of a new piezoelectric activity mechanism, suggesting that it may appear in a purely uniaxial ferroelectric in the form of correlation between lattice parameter and domains size. We observed this mechanism by time-resolved synchrotron Xray diffraction on SBN50 single crystals under alternating electric field. We have chosen SBN50 as a model uniaxial ferroelectric whose para-and ferroelectric phases are tetragonal The time-resolved X-ray diffraction experiment was performed using a custom-built stroboscopic data-acquisition system, which operates on the principle of a multi-channel analyser and qualifies for the investigation of repetitive processes down to the nanosecond time scale. The details of this technique are described elsewhere [4], [33][34][35] and briefly summarized in the supplemental materials. It has already been applied to the determination of small (~10 -4 Å) electric field induced bond distortions [36][37][38][39][40][41][42], the determination of