When you are taking a shit and have to exert extreme force in order to expel it out of your ass, thus causing more blood to rush to your face.
***Caution, this can break face capillaries.
Dave: "Hey, Unis, we can't go out to the bar tonight."
Unis: "Why is that?"
Dave: "I'm doing a red faced push and some of my capillaries might burst. I don't want my bro to see me like this."
Unis: "Well dear, you know I hear that Loreal matte make up works wonders!"