In what sense did God harden Pharaoh's heart?

Les explains that Pharaoh's heart became hardened because Pharaoh had already made a choice, lived with it, and when the next demand came, he chose wrong again and became hardened in the process. All God really did was put him on the spot. Every time God put him there with the demand to let Israel go, Pharaoh hardened his own heart, although Scripture says God did it. The same is true for mankind as a whole: when a heart is hardened, man does it to himself, yet it occurs under the sovereign volition of God. Their foolish heart was darkened as God's response to their own willful choosing. Even though God is sovereign and in total control, the exercise of free will remains ours, and as people choose to put God out of their thinking and think only in the human realm—secular humanism—their foolish heart is darkened.