It’s the whole ‘don’t think of a pink elephant’ idea.
The idea is that those deeper levels of mind communicate more so through imagery rather than in nuanced semantic language. So you can’t have a picture of a ‘not-rabbit’. In order to communicate ‘not rabbit’ in imagery, you have to put a rabbit there first and then remove it.
The preferable alternative is therefore to think of the positive or active conditions that you actually do want. Think of a banana tree, and voilà, you are also not thinking of a rabbit.
Makes a kind of basic sense to me. Subconscious or not, it’s probably better to get specific about what you really want. It’s a lot easier to hit the bullseye when you’re specifically aiming at the target, rather than vaguely attempting to ‘avoid things that are not the target’.
But I don’t see the need to adhere to it slavishly.