The hotspot comes from the information stored in the cursor font. The initial colors of a cursor are a black foreground and a white background (see X:Recolor-Cursor). The names of all cursor shapes are defined with the prefix XC: in `x11.scm'.
X:Create-Cursor creates and returns a cursor. The
foreground-color is used for the pixels set to 1 in the source,
and the background-color is used for the pixels set to 0. Both
source and mask must have depth one but can have any root. The
mask-pixmap defines the shape of the cursor. The pixels set to 1
in mask-pixmap define which source pixels are displayed, and the
pixels set to 0 define which pixels are ignored.