Using any saw — handsaw or power saw — requires careful setup in order to make sure your cut is clean and to avoid injury. Always secure a board with clamps to the cutting work surface before attempting to cut through a board with any saw.
If using a miter box, the box provides a snug place for the board. Still, fasten the miter box in place so the work and box do not move while cutting.
Do not hold the cut-off piece (the part of the board you are attempting to cut from the longer length) while using the saw. Always let the cut-off piece fall.