Plunge-cutting (cutting with tip of the saw, or boring) is an important technique. It solves many problems, and can actually make logging safer -- for example, when cutting a tree with a heavy lean.
However, it is also potentially dangerous, in that a kickback can easily result if it is not done properly.
Certainly, you don't want to try to push the bar straight in -- that will definitely cause a kickback. Start with the bar at right angles to the direction you eventually want to bore. There are two ways to do it; with a pulling chain (starting the cut with the top part of the bar nose) and with a pushing chain (starting the cut with the bottom part of the bar nose).
In this case, the bar will definitely want to kick -- but it will want to kick in to the tree. Make sure the bar can't kick past the tree. Let the tip cut in until a pocket is formed to hold the tip of the bar. As the pocket gets deeper, slowly pivot around until the bar is headed straight in.
Play a movie of cutting with a pushing chain.
In this case, we need to cut a pocket to hold the tip -- BUT if the top of the tip touches the top of the pocket, it will kick out. The trick is to slowly pull the saw toward you as you cut in the pocket so that the top of the nose does not touch the uncut wood. When the pocket is deep enough, you can pivot around and push the bar straight in.
Play a movie of cutting with a pulling chain.