There is plenty of scientific literature available on "instantaneous rigor" and "trismus" (locked jaw/clenching) to support the notion that Reeva's early jaw clenching was a direct result of the manner in which she died.
https://www.google.ca/?gfe_rd=ctrl&...ws_rd=cr#q=instantaneous+rigor+trismus&spell=
In fact there is at least one case report of rigor setting in on a patient BERFORE he/she died due to a severe vasoconstriction of the blood supply
http://www.google.ca/url?sa=t&rct=j...5IHQAQ&usg=AFQjCNFU6VZL_ql5awTfzYlyFEB2oYFbsA
Thanks, Murphy!