My impression is that the advice applies more generally than to just physical pain. My impression is that there's a lot of times when your body produces pain-like signals telling you to stop, when it's really in your interest to push through.

• Mental: overcoming akrasia.
• Emotional: doing things that scare you, but that will ultimately be in your interest.

And so, to use the emotional example, when I push past fear, I imagine it as weakness leaving my body. I imagine a) becoming a stronger person for having pushed past it and being more capable of pushing past it in the future. And b) I imagine it as weakness leaving the body in the sense of my pushing myself towards an end that is more preferable to me.