The measurement isn’t looking at the absolute amount of resistance, its looking at the resistance as measured many times per second and watching for a pattern. The pattern is your pulse. It generally looks like a sine wave, and we filter out measurements that dont fit that type of pattern. Sweat happens, and the total measurement changes over time, the wave might move up or down like a tide on a graph, but its still a wave. Because your pulse exists pretty much within a known range of frequencies. Generally, 40-200 beats per minute. We’re ok to just toss out the measurements that don’t make sense. Its okay if we miss a few - its gym equipment and you won’t notice anyway, we just keep saying the same thing until we can get good data again. If its been too long since we got good data, we just stop trying to measure it. Then you usually re-adjust your hands until the measurement starts working again.

Long story short, its not a perfect way to make a measurement but it works well enough that people can use it. We kinda know what we’re looking for, we know how to measure it, and the user will figure out where to put their hands to get it to display the numbers.