 MISC: When Doc demonstrates Einstein going back in time to Marty,  Doc motions for Marty to stand RIGHT in the tracks of the oncoming  Delorean, as if he was 100% sure that the time machine would work.  However, when the car does in fact disappear, Doc is very surprised  that it actually worked. Why would he put Marty right in front of  the car if he wasnt positive it would be a success?