When a person takes a crap and the water from the toilet splashes up onto the buttocks. Thus, it is like a "log ride" at an amusement park in that the "log" goes down into the water and you get splashed.
"I didn't enjoy my last crap because I got a log ride."
A Log Ride (sexual position) is when one person deficates (takes a shit) on another persons chest while in the sitting up position, then urinates (pisses) on them just above where they deficated. This reuslts in the feces (the shit, a.k.a. log) riding down the stream of urine (piss) to their stomache or further, much like a log going downstream.
"Chris said his wife let him give her a log ride the other night! That's were that stain on the carpet must have come from...."