  The detection of double black hole (BH+BH) mergers provides a unique
possibility to understand their physical properties and origin. To date, the
LIGO-Virgo-KAGRA network of high-frequency gravitational wave observatories
have announced the detection of more than 85 BH+BH merger events (Abbott et al.
2022a). An important diagnostic feature that can be extracted from the data is
the distribution of effective inspiral spins of the BHs. This distribution is
in clear tension with theoretical expectations from both an isolated binary
star origin, which traditionally predicts close-to aligned BH component spins
(Kalogera 2000; Farr et al. 2017), and formation via dynamical interactions in
dense stellar environments that predicts a symmetric distribution of effective
inspiral spins (Mandel & O'Shaughnessy 2010; Rodriguez et al. 2016b). Here it
is demonstrated that isolated binary evolution can convincingly explain the
observed data if BHs have their spin axis tossed during their formation process
in the core collapse of a massive star, similarly to the process evidently
acting in newborn neutron stars. BH formation without spin-axis tossing,
however, has difficulties reproducing the observed data - even if alignment of
spins prior to the second core collapse is disregarded. Based on simulations
with only a minimum of assumptions, constrains from empirical data can be made
on the spin magnitudes of the first- and second-born BHs, thereby serving to
better understand massive binary star evolution prior to the formation of BHs.
