The 5-wires from PP1 s ISP port are connected via this socket to the on board chip. The device can now be read and programmed. It is vital the ISP connector is not placed the wrong way around. Damage may result if it is. This is why a boxed header should be used.
Total isolation ensures greatest safety for both target system and the programmer hardware.
Simple, no electronic reengineering required. Suitable, small foot print SMD headers available. Low cost, only a single header and plug needs to be added. This method may use diodes, resistors and other components. The very general guide below assumes that no external power is applied to the target system.