  Switchbacks (SBs) are localized structures in the solar wind containing
deflections of the magnetic field direction relative to the background solar
wind magnetic field. The amplitudes of the magnetic field deflection angles for
different SBs vary from ~40 to ~160-170 degrees. Alignment of the perturbations
of the magnetic field and the bulk solar wind velocity is observed inside SBs
and causes spiky enhancements of the radial bulk velocity inside SBs. We have
investigated the deviations of SB perturbations from Alfv\'enicity by
evaluating the distribution of the parameter defined as the ratio of the
parallel to $\Delta\vec{B}$ component of $\Delta\vec{V}$ to $\Delta\vec{V}_A =
\Delta\vec{B}/4\pi n_i m_i$ inside SBs, i.e. $\alpha=V_{||}/|\Delta\vec{V}_A|$
( $\alpha=\Delta\vec{V}/|\Delta\vec{V}_A|$ when $\Delta\vec{V}~\Delta\vec{B}$),
which quantifies the deviation of the perturbation from an Alfv\'enic one.
Based on Parker Solar Probe (PSP) observations, we show that $\alpha$ inside
SBs has systematically lower values than it has in the pristine solar wind:
$\alpha$ inside SBs observed during PSP Encounter 1 were distributed in a range
of 0.2-0.9. The upper limit on $\alpha$ is constrained by the requirement that
the jump in velocity across the switchback boundary be less than the local
Alfv\'en speed. This prevents the onset of shear flow instabilities. The
consequence is that the perturbation of the proton bulk velocity in SBs with
deflection greater than 60 degrees cannot reach $\alpha=1$ (the Alfv\'enicity
condition) and the highest possible $\alpha$ for a SB with the full reversal of
B is 0.5. These results have consequences for the interpretation of switchbacks
as large amplitude Alfv\'en waves.
