The "Stranger" is when you sit on your hand until it goes numb, then you jack off, so it feels like a handjob from someone else.
"Stranger on the rocks" is the same thing, but you numb your hand in ice, so it's all cold on your Johnson.
My favorite, the "Strangers on the rocks in the night", is when you and a friend do a stranger on the rocks to each other, so it's like getting a handjob from someone else from someone else.