In this series the 'Indians' (if I can use that term as shorthand*) take their revenge in a variety of ways and this urination humiliation is one of the milder ones. If you've read Ulf's blog you will know that he has a keen political awareness and there's a fairly unambiguous message here conveyed by the Indian's heroic pose and the fiery background, succinctly symbolising the erupting anger of his people.
Ultimately though in this picture political message loses out to erotic entertainment. This Indian's nordic features and muscularity seem less than authentic and, for a gay audience, his enlarged cock puts him into the category of hunted trophy rather than feared, superior being. Likewise, the piss spray which he is unleashing probably dilutes(!) his outrage for us rather than amplifying it. If you invert the image you'll see his victim's expression is almost blissful too!
In this picture Ulf invokes the terrifying, scalping-renegade stereotype much beloved of white literature and films but thankfully from my point of view he spares us the blood and gore, contenting himself with posing the threat and capturing the young victim's dismay which he does rather impressively. Although this series of images has a somewhat 'staged' feel about it, you can see Ulf is striving here to make his characters interact, both dramatically and erotically.