When you're anally penetrating a female with your erect penis and, in your dominance and glory, decide to release a small amount of urine into her asshole. After all, why not?
"I was banging that bitch I met in the bar last night and was balls-deep inside her ass when she thought I came. She asked "Was it good for you?", and I replied "Not yet, you dirty whore! That was just a Mexican leak! Now shut up and hop back on my dick."
When, after drinking a significant amount of liquid -e.g. coffee or beer- over the course of a few hours, you anally penetrate a female's asshole with your erect penis and proceed to urinate until your bladder has been emptied.
If the amount of urine expelled into the female's colon is any less than one standard pint, the act is then referred to as a Mexican Leak.
Guy 1: "My wife is a fat whore who won't stop eating cheese. She's constipated for the 5th time this month, that disgusting bitch."
Guy 2: "Just buy her some Mexican food and it ought to clear her right out. Better yet, just give her a Mexican enema."