If the solder well is empty or insufficiently filled by solder, the resistance between pins 1 and 3 (+V and 0 V respectively) will be around 1.6 to 2.0 MΩ and the on-board voltage regulator will be enabled. If the well has been properly filled with solder the resistance across pins 1 and 3 will be around 1.2 KΩ and the on-board voltage regulator will be disabled.