Split weight position never occurs when actually performing most swing dance figures - it's an artificial static position usefull only for explaining other aspects of the figure, such as step placement or rise and fall.

In the swing dances, you must never let a leg reach in front of the body - to do so would be to put it in your partner's space. Thus CBM occurs as you send the body into the step, unlike in tango where you twist the body, reach the leg, and then move onto it.

It's a common fallacy to believe that you will get a bigger step by sending your leg ahead of your body on step one; in fact, all this does it put you off balance, making you place step two - the one that should be big as the swing uncoils - too soon, which is another way of saying too small. Step one is a preparatory action only - it's step two that goes somewhere.