Like 'back seat driving', back seat lifing means a more knowledgable person will want to help the stuggling person with all decisions, as they clearly have no idea of what they are doing. A back seat lifer has been there and done that many times before and just wants to pass on his immense knowledge of everything.
Man walks in holding a beer, another man back seat lifing says "don't hold it like that, it should be held like this"
Man sharpening a pencil, another man back seat lifing says "don't sharpen it like that, it should be sharpened like this"
Man standing minding his own business man back seat lifing says "don't stand there like that, you should be standing like this"