Extend your arms and fists out as if you're gripping some bars, then move your arms wider apart while keeping your wrists straight. You'll notice your hands just naturally twist outward. Back sweep is made to accommodate that, so that you aren't straining your wrists with wider bars. So it's only really needed for bars 21 or 22 wide and up, unless you're really small. But some people still prefer it with thinner bars anyways. It doesn't really make any tricks easier, just makes it more comfortable.
Which I guess in turn does make things easier, so w/e haha.