From what i understand Itchigo just used a Quincy technique. And since Bach want's him to join it would seem that that's the reason they where observing and testing Itchigo. They know he has either the ability to learn/absorb Quincy techniques or he has Quincy inheritance (from his mother maybe like some speculated). That's probably how he escaped. He awakened some Quincy power and escaped.

Anyways shouldn't analyse it further as it just make less sense. Better to just go with the flow.