A splat is a stern squirt against a rock or other fixed object in the water which has a current moving past it.
Splatting requires good boat control in the vertical dimension, and a willingness to undergo temporary pins against the rock or wall when the move fails.
Get creative: splat pillars, your friends, anything. You can get the bow up out of the water to start the splat by either a pry/stern squirt or by a forward stroke/stern slice. The key is timing. Judge the current speed and how long it will take you to approach the object. Get your boat not quite sideways to the object a fair ways upstream, and get ready to jam your stern down just a few feet from the splat. If you can get the bow up enough, the water will help drive you near vertical. Sometimes even getting the bow up just a few inches off the water will let you slide up the rock into a sweet splat.
Your technique will improve with practice, but the best splatting boats are the same boats that stern squirt easily.
The best splat objects are not quite vertical but tilted slightly downstream. Vertical objects have a tendency to drive you vertical too, and then your body weight pulls you backwards and down. (This is the perfect time for a screw up).
Bow splats are incredibly cool too. Basically, you paddle toward a splattable rock and throw a half of a flatwater cartwheel and then swivel your hips so that the bottom of the boat will contact the rock. Make sure you remember which way to spin or you will be visiting your dentist for lost teeth if you manage a face splat.