when activated it will force Core32 to bootup in Bootload mode, doesnt matter how buggy your progam is…

you need:
2x wires
2x 2Pin-Header
On-OFF Switch

solder this like shown here:

connect it to following pins of CORE32F4:

you maybe not need this, but when you need it, its good to have such switch on the backside of your midibox (@least when uploading, buggy betas, or you code your own program…)
greetings - Phatline