: A battery maintainer is critical during writing to prevent voltage drops that could "brick" (permanently damage) the ECU. Getting Started Guide Downloads - PCMflash
Boot mode (also known as recovery mode or BDM over OBD) allows tuners to access locked or corrupted ECUs. Version 1.21 refines the timing parameters for entering boot mode on and Siemens SID807 ECUs, reducing the failure rate from 15% to under 2% according to user reports.
If you find yourself outgrowing version 1.21, the natural progression is: