The flat snap mouse trap consists of five parts, each carrying a function that is necessary for the trap to catch mice: platform, striker (hammer), spring, holding bar (holds the striker in set position), catch (arrests the holding bar in set position and carries the bait).
|Figure 2-2 from Behe (Darwin's Black Box, p. 46)|
If any of these parts is missing, the trap will not function as a trap (irreducibly complex). So far the argument is correct.
3. Conclusion (absurd because it contradicts the historical evidence):
Hence the trap cannot have a precursor working with one part less.
4. Contrary evidence:
The direct precursor of current flat snap traps (the patent from which they all derived) had one part less and was working perfectly (catching mice, economic success story etc).
5. Resolution:Look at the drawing below and you'll see that the precursor had one part that was carrying two functions. The spring and the striker were formed of one wire (starting at #3, twisting through #4, passing along #2 and 8, running through the spring and ending in #5).
|Patent drawing from Hooker (1894)|
Hence all the functions necessary for the trap to work were carried by four parts. Behe's conclusion (3.) is false, because it tacitly assumes that one part carries one function and that one part less means one function missing.