Now that I think of it, I remember accidentally making pen strokes with some empty pockets inside, a bit similar to those in 'y' and 'm' above. I think it had something to do with the shape of the nib and the way the ink tends to stay in the slit.
I managed to find a photo of my very first experiment with a feather, turns out it was even without ink, just using water and a practice pad for Chinese characters (one that turns black when wet and turns back grey as it dries up). As you can see, there is a bit of white inside the principal stroke of 'h'. I think, judging by the width of the stroke, this happened not because of excessive pressure or angle that would split the tip, but by using a little less pressure or a lighter/quicker stroke with the tip not fully touching the pad.
I managed to find a photo of my very first experiment with a feather, turns out it was even without ink, just using water and a practice pad for Chinese characters (one that turns black when wet and turns back grey as it dries up). As you can see, there is a bit of white inside the principal stroke of 'h'. I think, judging by the width of the stroke, this happened not because of excessive pressure or angle that would split the tip, but by using a little less pressure or a lighter/quicker stroke with the tip not fully touching the pad.