FLD means the ability to move blocks of graphics vertically on the screen. More precisely FLD operates on char-lines (8 pixels high) as its smallest element. With FLD you can make gaps between char-lines, thus the graphics below the FLD will be pushed down on the screen.
Here is a short piece of code that shows a very basic FLD-effect. FLD does not require exact timing and doesn't even have to be updated each rasterline (only every ~6:th rasterline is ok). FLD can even be done with periodic interrupts instead of looped code that just wastes all the precious rastertime. That exercise however, i leave to you..
sei loop1 bit $d011 ; Wait for new frame bpl *-3 bit $d011 bmi *-3 lda #$1b ; Set y-scroll to normal position (because we do FLD later on..) sta $d011 jsr CalcNumLines ; Call sinus substitute routine lda #$40 ; Wait for position where we want FLD to start cmp $d012 bne *-3 ldx NumFLDLines beq loop1 ; Skip if we want 0 lines FLD loop2 lda $d012 ; Wait for beginning of next line cmp $d012 beq *-3 clc ; Do one line of FLD lda $d011 adc #1 and #7 ora #$18 sta $d011 dex ; Decrease counter bne loop2 ; Branch if counter not 0 jmp loop1 ; Next frame CalcNumLines lda #0 bpl *+4 eor #$ff lsr lsr lsr sta NumFLDLines inc CalcNumLines+1 rts NumFLDLines .byte 0