The First Six is the first line of Retreat, a yin line in a yang position at the very bottom of the lower trigram Gen (Mountain). 'The tail in retreat' means being at the tail end of a withdrawal — the last to leave, the most exposed. When everyone else has already pulled back, you're still caught in the danger zone.
This line paints a vivid picture: imagine a herd of deer fleeing from a predator. The one at the very back, the slowest, the 'tail' — that's you. The lion is closing in. 'This is dangerous' — understatement of the century. 'One must not wish to undertake anything' — your only option is to stay absolutely still, play dead if you must, and pray the danger passes.
As the card depicts — a figure caught at the rear of a retreating group, danger closing in from behind. You have missed the optimal moment for withdrawal. Now is not the time for heroics or counterattacks — it's time for damage control and absolute stillness.