He provides a great summary of Bahnsen's presuppositional approach, so I'll leave you to his analysis. From my perspective, what Bahnsen did most effectively was to show how Stein's presupposition that a debate itself over the truth or falsehood of an idea was possible and worthwhile demonstrated that Stein had already accepted Bahnsen's theistic worldview. In other words, Stein had no valid atheistic explanation for the laws of reason and logic that governed the debate and that he employed in it.
If you're interested in learning more about apologetics, Bob's post is a pretty good place to start, and Bahnsen's approach is powerful, even though I am still convinced that no one will ever come to believe in the God of the Bible apart from a direct redemptive action on the part of the Holy Spirit. For more discussion on that topic, tune in to this interview of Carl Trueman by Mark Dever. This part of the discussion begins 34:45 into the interview, and at 36:49 Trueman makes this excellent statement:
It's patently obvious that it doesn't matter how convincing an argument you offer to somebody. They're never going to believe on the basis of the argument you offer to them.Be sure to listen for the full context, which continues through minute 39.