I think they may originally have planned to charge Sollecito with obstructing justice by lying in his earlier statements to police (i.e. those before the night of 5th November, in which he was clear that he and Knox had spent the entire evening and night together in his apartment).
I think that on 5th November, the police - by now clear in their own minds of the "truth" - planned to psychologically bombard Sollecito until he confessed to them that he had been lying all along, and that Knox had in fact left his apartment at some point on the evening of the murder. They would then use this in their plan to break Knox in turn.
But as it turned out, they couldn't really break Sollecito (in essence because their "truth" about the crime was in fact totally wrong). The best they were able to manage was to get him confused about dates between Halloween (31st October) and the following night, the night of the murder (1st November). Eventually they got him to say a vague statement along the lines that he couldn't be certain that Knox had been with him all evening/night, though in my view it's abundantly clear that this was the product of a confused, tired and coerced man.
However, that would have to do for the police, and they obviously felt it was enough to use in part 2 of their plan. They duly brought Knox in for interview (I believe the original plan was probably to send the arrest squad over to Sollecito's place to grab her with all lights and sirens, but she thwarted that by accompanying Sollecito to the police HQ), and told her that Sollecito was no longer supporting her claim to have spent the whole evening/night with him at his apartment.
As I said above and in a previous post, I think the plan was to charge Sollecito with obstructing justice. But I think the police/PM were then thrown when - in front of Matteini - he strongly reiterated his prior statement that he and Knox were indeed together in his apartment for all of that evening and night. And then they were told by the idiots Rinaldi et al that Sollecito's trainer print had been positively identified in Kercher's blood inside Kercher's room. Suddenly it made sense to the police and PM: Sollecito had never been lying to protect Knox - he'd been lying to protect himself and Knox together. He'd actively participated in the murder alongside Knox and the third man.