I'm not entirely sure I understand what you're asking. Here's the full sequence as reported in a 1987 report on the incident:

Before the accident, the manual arming pin in each of the bombs was in place. Although the pins required horizontal movement for extraction, they were both on a lanyard to allow the crew to pull them from the cockpit. During the breakup, the aircraft experienced structural distortion and torsion in the weapons bay sufficient to pull the pin from one of the bombs, thus arming the Bisch generator. The Bisch generator then provided internal power to the bomb when the pullout cable was extracted by the bomb falling from the weapons bay. The operation of the baroswitch arming system, parachute deployment, timer operation, low and high voltage thermal batteries activation, and delivery of the fire signal at the impact by the crush switch all followed as a natural consequence of the bombing falling free with an armed Bisch generator. The nonoperation of the cockpit-controlled ready-safe switch prevented nuclear detonation of the bomb. The other bomb, which free-fell, experienced HE detonation upon impact. One of the secondary subassemblies was not recovered.

The two switches of note are the "arming pin" (a pull-out switch), which in theory ought not have activated but happened to (because of the torsion of the plane breakup). Once that was stared, the detonation sequence continued as it ought to have until it his the ready-safe switch, which was in the "safe" position (as it ought to have been), which stopped the sequence from continuing to the actual detonation.

I have written a bit more on this switch and incident here, which links to said report, a film Sandia made about it, etc.