As the sequence starts, they are crossing the street. A flying-train is going overhead, and Clark is checking it out with his super-vision.
Clark "accidentally" blunders into the man walking in the opposite direction, knocking him down.
The guy shouts at Clark and Lois as the muffler falls right where his head would have been without Clark bumping into him.
Basically, Clark saved the guy by predicting the danger, and delaying him, while simultaneously backing up his "clumsy Clark" secret identity.
Interesting note: I was confused by this sequence, but my mum, who hardly ever reads comics understood it first time.