In my experience, the perfect takeoff position for a loop jump is with the free foot resting gently on the ice, heel of free foot in front of the toe of the takeoff foot. This forces you to bend your takeoff knee, keep the free hip closed and get your chest over knee over toe before takeoff. The free foot on the ice does not help you in terms of takeoff power because it's too far away and not under you. If you shift your weight and end up taking off from the "free foot" instead of the foot you're supposed to take off from, I would imagine they'd call it as a salchow instead of a loop (LBI takeoff instead of RBO takeoff).
However, whenever you put your free foot down *between* jumps, it is considered a two-footing of the first jump (or a stroke between jumps, which also isn't allowed) and it negates any jump that comes after that.