To boot in recovery mode on a LG G2, you need to do the following steps:
- Power off your phone. Don't just restart, but perform a normal power off
- Wait for a few seconds
<Power>buttons simultaneously until the LG logo is shown
<Power>buttons simultaneously again until you get the firmware reset screen
- Press the
<Power>button again to start the reset
This is useful when you want to flash a firmware via e.g. TWRP.