In Japan long rips from a few inches to 20 feet or more are done with the board raised up about a foot or higher off the floor on blocks or shorter sawyer stands. A good dark sumi ink line helps and a sharp, perfectly tuned saw is imperative. All the cutting is done bending over and pulling the saw handle up to your chest as you are walking backwards on the top of the lumber. Out of respect for the wood and to keep it clean I take off my shoes and in Japan you are trained to do this. If you are ripping a narrow board you can have a second board alongside for stability, but your foot pressure near the cut prevents chattering. The cut line does not progress between your feet but out beyond your forward foot, and you shuffle backwards instead of walking backwards. Have a good ink line to follow and also ink the endgrain clearly with your vertical line. You will need to blow off the sawdust as you go along with strong puffs.
To begin the cut it is good practice to while looking down, lean over the end of the stock and at the bottom/underside of your drawn line on the endgrain, accurately nip the bottom/edge of the board on the endgrain surface with a shallow cut along the side of the line you are going to follow. This is most easily done with the smaller crosscut teeth on your ryoba, nip the bottom edge then nip the tip or corner of the top edge endgrain, then go back and nip the bottom edge a bit more so you begin to build the cut line as it develops on the endgrain up from bottom to top until they meet in the middle of the end for the board. This makes a shallow accurate cut line on the end of the lumber. In this way your first full power strokes will be perfectly vertical while riding in the shallow accurate cut line you just made. Don’t get too anxious with the first strokes, keep them light and accurate.
Remember that your power strokes are on the pull and that your most accurate nip strokes are also on the pull. On heavy timbers this endgrain set-up is obvious, but even on sheeting or thin stock it is good to always follow the same method. This first vertical power cut stroke from the underside to the topside at the end of the lumber has to be true or you are screwed to whole way. It is not a bad idea to have a line snapped on the underside of the board too so you can check for drift in your cut as you go along by flipping the board over. If you cut consistently drifts left or right then you saw set is off or you have a missing tooth or a broken tooth tip or two. With a well tuned saw you can cut forever on one side of the line with no problem of drift, even with your eyes closed. Don’t cut down the middle of the line, bad form. Cut on the waste side so you can plane or finish the cut surface later to dimension the wood. For westerners with long legs you find that a lot of japanese hand work is hard on the back because so much is done at floor level or bending over, the tools are designed this way, but with a developed sawing stroke ripping this way is very accurate and much of your labor is taken up with the larger leg muscles.
A couple words of caution. Knots in the wood are hard and can act like stone to fine hard saw blades and will chip or break teeth. Good and great saws break easier then cheap saws as a general rule. Look your lumber over and know where all the knots are, so when you approach a knot area slow way down like from 30 miles an hour down to 3 mph in the vacinity of knots and when sawing through knots proceed at 1 mph speed and only with the rip teeth. Even when cross cutting a board, when you need to saw through a knot, use a rip saw and go slow to save yourself much anguish. Crosscut teeth break easier, so use the rip teeth in knots.
Also in general saw slow, let the saw do the work, don’t torque the saw handle to curve the cut and most of all do not flex or bend your blade in the passive down stroke, you can easily break off a corner of your saw or even break it in half. Again, good saws and really great handmade saws are very hard and brittle and break easily, don’t tempt fate. Alx