As stated there are many variables to the line of fire. As a general rule it is a cone shape coming off the front (rim) and back (chuck, face plate or holding method) of a bowl/piece all the way around the rotation of said piece. I say general rule since ricocheting and fluttering could become a factor. The faster your spinning generally makes that cone a bit tighter since the velocity flings it quicker thus straighter line from the holding method to the rim as referenced by what Robo shows in his video. The thinner (wall thickness) the bowl becomes the cone shape becomes more like a funnel shape. This is because velocity will want to throw it straight however you have the effect of air catching the lighter piece or the piece shatters which can make the piece wobble/flutter more (watch a bird glide with their wings set and how winds can make them wobble/flutter and change direction a bit in flight). These things happen fast and most times we as humans wince and close our eyes as its happening.
Other variables are the heavier the piece, if it comes out of your holding method, the more likely it will be drop down from a 3 o'clock to 9 o'clock area. In other words it will drop or go down rather than up due to the weight (again reference Robo's video). Don't be fooled though because that is generally and not always. In Robo's video his came loose at a bark inclusion or damage from a storm and not from his holding method. Those types of objects can come free in any spot around the rotation meaning if it came free at the 12:00 point in rotation it could very well come right at your head if you are standing in that cone line of fire. As everyone says the best method is do not stand in the line of fire as the best option for safety. Just try to understand some forces from weight, size, speed and etc. to understand the higher risk "line of fire" areas for the piece you're turning and where you are in the turning process. This is hard for newer turners so I agree this subject could be covered much better in books and demonstrations. Those who have been turning longer understand these areas of expectations of where something most likely will go if it comes loose, unfortunately by experience from it happening and not by some divine knowledge.