This is a myth! Filing backwards and forwards (or see-sawing) won’t harm the natural nail tip as long as you use the correct grit (abrasive level) and only apply a light, gentle downward pressure, letting the file do the work for you. The ideal grit for the natural nail is 240, basically the higher the number the more gentle or softer the file.
Don’t use a file with a grit below 180 and avoid using metal files on your natural nail because it can cause damage, for example the layers of the nail can delaminate or come apart and start to peel, no matter how you file. The backwards and forwards filing method was considered bad practice in the past and some colleges are still teaching this myth.
However the latest research by some of the top scientists in our industry have busted this myth. Because the nail cells are laid down in a brickwork type pattern, the backwards and forwards motion can actually push them closer together and help to strengthen the nail where as only filing in one direction can destabilise the bricks and result in the nail tips peeling or splitting.