FPD means the ability move a single pixel-line of graphics vertically on the screen. FPD can visually be compared to FLD, only that FPD is 8 times more beautiful, but technically it's quite different. You can also simulate FLD with an FPD-effect.
The invention of FPD probably origins from the crap gfx you get when you do a line-cruncher. Linecrunchers are mostly used to move big areas of bitmap-gfx on the screen, where as copying would take far too much time. The crunched lines stack up above the bitmap-area that you actually want to display, but those crunched lines can of course be used for something as well, and FPD is the effect that allows for here. By introducing gaps between the crunched lines (just like in FLD), you can control every individual pixel-line distance from the previous pixel-line.
Here follows a code example, which is based on the linecrunch example. Perhaps you'll better understand all details if you tried that one first. This routine does not really take every pixel-line and do desired amount of FLD on it, but rather it reads one byte per line from a table that tells wether this line should be displayed or not. If the line should not be displayed then a value is stored to $d011 that does not trigger any badline, linecrunch or anything, i call it an FLD-line. If a line should be displayed, then a value that triggers linecrunch is stored to $d011 at the right timing.
sei loop1 bit $d011 ; Wait for new frame bpl *-3 bit $d011 bmi *-3 lda #$3b ; Set y-scroll to normal position sta $d011 jsr SetupFPDTable ; Nice FPD effect lda #$59 ; Wait for position where we want FPD to start cmp $d012 bne *-3 ldy #10 ; Wait one more line.. dey bne *-1 nop nop cmp $d012 ; ..and make a bit more stabel raster bne *+5 bit 0 nop ldx #0 ; Clear counter loop2 ldy #5 ; Wait some cycles dey bne *-1 nop nop nop lda FPDTable,x ; Load 0 or 1 from table, where 0 will skip a line with FLD ; and 1 will draw a line using linecrunch lsr ; 0 or 1 moves into carry flag lda $d012 adc #7 ; Add 7 or 8 depending on carry and #7 ora #$38 sta $d011 ; Do one line of FLD or LineCrunch nop ; Wait some more cycles so that the whole loop ends up on 63 cycles (= one PAL raster line) inx cpx #100 beq loop1 ; Exit if end is reached jmp loop2 ; Otherwise loop SetupFPDTable lda #0 ; First clear the table sta YPos ldy #0 sta FPDTable,y iny bne *-4 lda #0 ; Increase the starting value inc *-1 asl sta AddVal ldy #0 ; This loop will insert 16 1:s into the table.. ; At those positions the graphics will be displayed SFT_1 lda AddVal clc adc #10 sta AddVal bpl *+4 eor #$ff lsr lsr lsr lsr sec adc YPos sta YPos tax lda #1 sta FPDTable,x iny cpy #16 bcc SFT_1 rts YPos .byte 0 AddVal .byte 0 .align $100 ; Align the table to a new page, this way lda FPDTable,x always takes 4 cycles. FPDTable .dsb 100,0 ; Reserve 100 bytes for the table