Just about any SA handgun will not fire if pushed up against a surface, it puts the gun out of battery. Just holding the slide may stop the gun from cycling but it will not put the gun out of battery. I am just guessing but I would say DW had the gun pressed against the door panel, either before MB tried to grab it or after he tried to grab it. I grab any of my guns, by the barrel, and I will have my hand on the spring shroud as well as the slide. Now either DW is just confused or MB was actually trying to block the gun, not grab it, or DW already had the gun drawn as MB approached pushed against the door panel.
I read his testimony and it is an embarrassment for a police officer. He indulges in hyperbole, and his version of the events leave no doubt he is incompetent.
DW testified that;
He was in a dangerous neighborhood
He noticed that MB was a very large person before any contact
That initial contact MB used offensive profanity towards DW
Darren Wilson called MB to his vehicle with the window down!