The transition from the first equation to the second equation here represents what happens in the limit as you let P (the number of available processors) go to infinity. This shows that the upper bound of program speedup here is capped at 2.

Given the fact that we are supposed to be talking about Amdahl's Law in this section, we note that S = 0.5 because we are letting the entire second half (or 0.5) of the program be sequential. This is an alternative way we could have derived that the speedup is capped at 2.