  We report on the first detailed study of the mechanisms involved in knockout
reactions, via a coincidence measurement of the residue and fast proton in
one-proton knockout reactions, using the S800 spectrograph in combination with
the HiRA detector array at the NSCL. Results on the reactions
$^9$Be($^9$C,$^8$B+X)Y and $^9$Be($^8$B,$^7$Be+X)Y are presented. They are
compared with theoretical predictions for both the diffraction and stripping
reaction mechanisms, as calculated in the eikonal model. The data shows a clear
distinction between the two reaction mechanisms, and the observed respective
proportions are very well reproduced by the reaction theory. This agreement
supports the results of knockout reaction analyses and their applications to
the spectroscopy of rare isotopes.
