The knewl post is usually anchored to the floor or to from face of the knee wall.
What I've done was put 4, 1/4" lag bolts into the post from the basement. I positioned the post on the floor and drew an outline on the floor. Then I marked 4 'spots' on the floor for where I want the bolts to go. THEN I drilled down. Then I took a metal plate not much wider than the post, set it over the drawing on the floor, went into the basement to mark 4 holes on the plate, and drilled out the 4 holes. Then I set the post back over the drawing on the floor, and went down the basement with the plate and screwed in the bolts. So, you have bolts through a metal plate going up through the subfloor and into the post. The upper part of the post is anchored to the handrail. It's best to somehow secure the post from the SIDE as well. As a matter of fact, I think this is a necessary precaution. It stablizes the post even more.
Best to ya and hope this helps. Oh, my method works only if the knewl post is BETWEEN 2 joists! Also, be sure to mark which side of the plate is UP and which side of the post AND the plate face the, say, front of the house. This way you everything lines up and isn't upside down or improperly 'oriented' from your initial drawing and drilling.