"Mind-body connection" is used so loosely that it's come to mean almost nothing. Underneath the vagueness there's something specific and well established: physical states and mental states inform each other constantly, in both directions, whether or not you're paying attention.
The uncontroversial version
Anxiety produces a faster heart rate. That's not a metaphor.
And the reverse: a rapid heart rate from any cause can produce anxiety. Posture affects mood. Hunger affects judgement. Poor sleep affects emotional regulation measurably.
None of this is alternative. It's ordinary physiology, and it's the real content of the phrase.
The part most people miss
Most of us are poor at reading our own physical state. Tension that's been there for hours goes unnoticed. People routinely discover in a quiet moment that their jaw has been clenched since morning.
The capacity to notice internal signals varies between individuals, and it can be developed. That's much of what body-based practices train — not relaxation exactly, but noticing.
Why noticing matters
You can't address a state you haven't registered. Someone who notices they're bracing at eleven in the morning has options that someone who notices at eleven at night does not.
Over time, people who practise this tend to catch things earlier — the tightening before the headache, the breath change before the anxiety, the tiredness before it becomes depletion.
Where claims overreach
The honest version stops well short of where the phrase is usually taken. Body practices do not cure disease. Emotions do not cause cancer. Posture is not a treatment for depression. Anyone connecting those dots confidently is going far beyond the evidence.
What's supportable: physical and mental states influence each other, attending to the physical is a genuine route into the mental, and this is useful. That's enough without inflating it.
What practices do with it
Yoga, qigong and similar traditions all work this way — using attention to breath, movement and sensation as a way into states that are hard to reach by thinking.
You don't have to accept any traditional framework to find the method useful. The mechanism people actually report is mundane and real: slow down, pay attention, notice what you're doing with your body, change it.