Bioelectrical impedance devices have a real weakness in relation to hydration. If you can maintain your hydration levels, then they could at least track your gain and loss. But doing anything that disrupts that balance, and that is so easy to do, will throw off the measurement.
Adding in a pulse fast will cause a disruption in your hydration, and then throwing in a cheat meal on top of that could throw a little shock to your system to throw off your hydration levels.
No method is perfect, but I would put impedance at the bottom of the list.
I have been using skin fold measurements myself, (cheap and can be done at home,) but while being more accurate (if done right that is,) is still not perfect. And while I have been putting up the numbers here, and doing the math to show the change in body fat and lean body mass levels, I am aware that these numbers are not going to be accurate. But the accuracy I am actually looking for is that the numbers are moving in a positive direction.
But even then the "official" number should only be done every 2 weeks. You can check it sooner if you want to, but don't use the number, and make changes until the "official" 2nd week number comes in. This keeps you from freaking out over minor fluctuations over a couple of days.