Originally Posted by Kapusta

Resistance in the pedaling may increase (sort of, but it will likely be too late) but not in the shifting. If you have the capacity for your 2nd biggest cog, but not your first, then you will shift into the 2nd biggest no problem. And you will most likely START to shift into the biggest cog with no problem, as you are barely using any more slack. The problem comes as the chain on the larger cog starts making its way around the cassette, slowly taking up slack as it goes. Once it reaches the point where there is no more slack, something has to give. And the problem is that at that point when you are pedaling, you have a LOT of leverage against the chain stretching.