You insulate the motherboard. The aim is to allow only die contact the cup your N2 is in.
There are still problems like cpu/board/components not working due to low temperature or condensation. Generally they are solved by better insulation and trying different components until you find ones that work.
Overclocking is 25% cooling and 75% picking parts that can operate at low temperatures and high frequencies.
Don't forget the crazy binning for ram sticks, cpu dies, and graphics cards. There's a reason all the top guys have major sponsorships, it takes a lot of money to find the top 1% of parts.
