When Shen Wulong was explaining things to Duncan, he didn't have that much understanding of cultivation. So, he wasn't able to ask such pertinent questions. But, now that his cultivation and understanding of concepts have risen, his mind was beginning to churn with new ideas.
It was not as if Duncan was the first to think of such things. But, most cultivators use complementary concepts of two daos to create some domain. Like combining the concept of revolution and pressure to form a whirlpool, or combining rotation and speed to form a penetration point.
Duncan however has never come into touch with other concepts until now. So, he was only left with the option of two concepts he knew. As they say, a young calf knows no fear.
Squinting a little, Duncan smiled eagerly, "Let's try and see what happens."