Although this takes consistent reminding and practice, the payoff is a much richer, fuller and more blended sound.
While placing the hands over the bottom of your ribcage on either side, take in a long, slow, deep breath and feel the expansion.
As you sing a note, press in slightly with the hands, and resist that pressure by holding the ribcage out – using your intercostal muscles to press outward. It’s not necessary to actually know anything about these muscles. The only thought that’s required is that you’re resisting the pressure from your hands.
Once you have a sense of this, drop the hands and just focus on leaving the ribs expanded outward, as long as possible, as you sing.
There’s also an additional bonus – and that is that when you begin a note with the ribcage expanded it’s much easier to start a vowel without a glottal bump.