to feign an advance along a main route while secretly taking a detour to strike by surprise; using a deceptive front to cover a hidden agenda

Literally meaning to repair the plank road by day while secretly crossing at Chencang, this idiom describes using a diversion to hide one's true intentions or activities.

pile trestle; a type of plank road or bridge structure built along cliffs and supported by vertical wooden piles

the Qinling plank road to Shu; a historical mountain road connecting Shaanxi to Sichuan