This switches the effect from vertical slits to horizontal slits. Vertical uses left and right halves. Horizontal uses top and bottom halves.
More frames create a longer time span. The maximum buffer depends on slit width so the full time band still fits on screen.
The centered time band is built from buffered slits. Vertical mode runs oldest-to-newest from left to right. Horizontal mode runs oldest-to-newest from top to bottom.