Long stretches of uninterrupted thought are required to correctly partition the problem, which is a hard problem as the number of possible partitions is roughly exponential to the number of moving parts. Once the problem is optimally partitioned, understanding the solution is a linear effort. Hence, effort required to understand the problem is not indicative of the effort required to understand the solution. Now, for a new person to understand why this partitioning was preferred to all others requires one to go all the way back, and I worry can never actually be replicated, which is why old software projects often stagnate after the original architects depart.