Games are written on both sides of a floppy disk, and you have to turn it over to read the other side.
Its use depends on the emulator you're using.
The L3 and R3 buttons allow for the first one to eject the disk (L3) and for the second to change the side and read the new side (R3). So you have to press the combinaison L3 then R3 in a row to change the side.
The L1 and R1 buttons allow for the first one to eject the disk (R1) and for the second to change the side (L1). So you have to press the combination R1 then L1 then R1 (eject + swap + read) in a row to change the side.