A line popped out the other day in discussion of writing with others: "Writers are liars and fabricators, but not bullshitters." It was something more intuited than thought-out, and so it took a moment to word a more precise explanation.
Whatever kind of story we tell, it has to be grounded in some kind of truth of things. The most basic kind of truth is factual truth, which is how many kinds of mysteries and thrillers work. They exploit either some real-world factual truth, or our lack of observation about some in-universe set of facts. A good thriller or mystery never cheats.
With science fiction, and especially fantasy, there's more opportunity to cheat, but also more opportunity to get away with it. In such a story, you cheat by setting up rules about how things works, and then establish how the rules can be bent or broken. Maybe "cheating" is the wrong word, but that's probably how it plays out to the absolute purist: why set up the rules in the first place if you're just going to break them? But that's what makes a story interesting: not that we follow the rules gamely, but that we push at their implications until something gives and we end up somewhere new and wonderful.
An author who builds trust in their readers first, and then bends the rules, gets more leeway to bend. If you punch an emotional ticket with a story, you tend to stay with it more fiercely. If you trust the author to not abuse the way he deploys suspension of disbelief, you'll follow them off the cliff (and maybe to the other side of the canyon, a la Bugs Bunny blithely defying gravity).
How you cheat makes all the difference. You have to pave the way to the cheating, as it were; you have to show there's a buildup to something that will in time give. Just pulling it out of your back pocket as an afterthought never works. Maybe you can get away with something that snide if you're Agatha Christie, but even she couldn't get away with it all the time.
That's the difference between cheating and b.s.ing. The first is part of a larger plan of showing how the rules in the work are just the beginning. The second is a bluff when you try to get away with admitting you don't have a plan, but hey, we're all consenting adults here, right?