  Gyrochronology, a valuable tool for determining ages of low-mass stars where
other techniques fail, relies on accurate calibration. We present a sample of
327 wide ($>$$100$\,au) white dwarf + main sequence (WD + MS) binary systems.
Total ages of WDs are computed using all-sky survey photometry, Gaia
parallaxes, and current hydrogen atmosphere WD models. Using a magnetic braking
law calibrated against open clusters, along with assumptions about initial
conditions and angular momentum transport, we construct gyrochrones to predict
the rotation periods of the MS stars. Both data and models show that, near the
fully convective boundary, MS stars with WD ages up to 7.5\,Gyr experience a
rotation period increase by up to a factor of $\approx$$3$ within a
$<50\,\mathrm{K}$ effective temperature range. We suggest that rapid braking at
this boundary is driven by a sharp rise in the convective overturn timescale
($\tau_{\mathrm{cz}}$) caused by structural changes between partially and fully
convective stars and the $^3 \textrm{He}$ instability occurring at this
boundary. While the specific location in mass (or temperature) of this feature
varies with model physics, we argue that its existence remains consistent.
Stars along this feature exhibit rotation periods that can be mapped, within
1$\sigma$, to a range of gyrochrones spanning $\approx 6$\, Gyr. Due to current
temperature errors ($\simeq$$50\,\mathrm{K}$), this implies that a measured
rotation period cannot be uniquely associated to a single gyrochrone, implying
that gyrochronology may not be feasible for M dwarfs very close to the fully
convective boundary.
