[Update: May 2022] How to trick the heart rate monitor on your fitness tracker (NSC edition)

Disclaimer: The following information provided is for educational and experimentation purposes.

Recently I've seen a spike in interest over this topic based on Google search results. Hence I will give a quick summary on the latest garbage trackers given free to participants of the steps challenge.

Testing performed on AXTRO FIT (Season 4) & Axtro Fit 3 (Season 6)
Final results and conclusion:
Certain clear or reflective surfaces may induce trackers to record MVPA values, surprisingly consistently. Steps count sensitive to vibratory effects from simple rotary mechanisms such as fan motor without fail.

Don't throw that Xiaomi Powerbank 5000 yet...

If you have a Xiaomi Powerbank 5000 and find that it keeps flashing the full LEDs when charging but when discharging to a device, it instead flashes one LED, then it could be the wire for the temperature sensor has broken off the circuit board.

Open up the powerbank on the non USB port side with a blade and screwdriver then slide it out at the opposite end. You may find that the one or both of the two wires connected from the battery to the circuit board may be disconnected from its solder point. Solder them back then test it. If it works then put it back together.

This video shows where the wires are:

Getting into Recovery mode for Nexus 5

1. Hold Power button + Volume DOWN button to boot into the Bootloader
2. Press any volume buttons until "Recovery" is shown and press the Power button. 
3. Phone will reboot and show an Android with the exclamation point appearing above the stomach shows with "No Command" below.
4. While holding the Power button, press once on the Volume UP button and it will go into Recovery mode.