This is a reference to trying to walk straight (walk a line) and pass a field sobriety test, while drunk or intoxicated, in front of a Law Enforcement Officer.
A man passes a field sobriety test while drunk by walking in a straight line. By walking straight, he is released and free to go, even though he broke the law. He fooled the cop and does not have to go to jail.
A Shackteau is a humble, weather-beaten, structurally questionable shelter located in a spectacular or highly coveted place—Wales, Jackson Hole, Sun Valley, Crested Butte, coastal Maine, the Alps—where the building itself may be worth almost nothing, but the dirt, view, access, and mythology make it absurdly valuable.
In use:
Shackteâu - We thought it was an abandoned shed until the realtor called it a rare alpine Shackteâu with unobstructed views and listed it for $2million.