You need to slice the clip so that your starting point on the timeline is the end of the part you want reversed. In other words, if your clip is 1234567 and you want to show 45 backwards as 54, you have to split at 6 (the end of 5), then apply the -100% reverse starting there. If you split at 4 and then reverse, you'll see 32, since that comes before 4.

Think of it like a VCR. If you want to watch a section of a videotape from 30:00 to 32:00 backwards, you have to fast forward to 32:00 to begin with. If you start at 30:00 and play backwards you'll go to 29:59 next.