Why tilt your bow drill spindle? Generally speaking, drilling straight down into your hearth is a solid strategy, but some subtle adjustments to the angle at which you position the spindle can go a long way toward avoiding frustration and convincing difficult sets to produce embers.
When you first start drilling into a new divot, the dent that the spindle tip rides in is usually pretty shallow, and this can make it prone to jumping out at the notch. Tilting the top of the spindle toward your body angles the tip away from the notch and can help prevent this common problem.
Once the spindle is well seated, adjusting your spindle so that it sits approximately vertical and perpendicular to the hearth will make it easier to drill while you build the dust pile, as the friction will be concentrated on the tip of the spindle, making it easier to turn, and easier for you to work the bow.
You can often just add speed and pressure at this angle to ignite your dust pile, but you can get an extra advantage by tilting the top spindle slightly away from your body. This helps to better direct the hot dust into the notch, and also points the hot tip of the spindle directly into the dust pile.
Years ago a friend suggested to me that it is contact with the spindle tip that actually ignites the flammable dust, and this has proven to be a very valuable insight. I don’t know if this is always true, but looking at it this way has helped my success rate immensely.
I exaggerated the angles a bit to make them clear for demonstration purposes, but this isn’t watch making, so close enough will often get you there.
There are other reasons you may want to tilt your spindle that I didn’t cover here. I find it very useful for methods like the two-hole/keyhole hearth, no-notch hearth, utility drilling with a spindle, and managing issues with a set to direct forces away from weak points in the hearth when working with poor materials.
This is just as useful (if not more so) for the hand drill, where the margins for error are slimmer, and small adjustments are more likely to make the difference.