I have used a number of techniques for destemming leaf. I've destemmed with a rotary knife (pizza cutter style), a curved tactical knife, a chaveta and various ways of destemming by hand. Currently, I have settled on destemming by hand exclusively.
What I've found is that the case of the leaf is critical. I start with about 20-30 crispy-dry leaves. Held by the butt end of the stems, I mist these thoroughly with distilled water (distilled so that it won't clog my spray bottle over time). These are then carefully placed into a very large, transparent plastic bag, and closed with a twist of the plastic and a clothespin. Usually by the following day, the leaf can be felt to be entirely pliable.
The key to hand stemming appears to be putting the leaf in medium case, while the stem is still in low case (not dry). This allows the lamina to be held without breaking, and be torn cleanly from the stem by drawing the lamina along the direction of the stem, rather than pulling it away. I use about half the length of my index finger, together with the long edge of my thumb. If the stem is too moist, then it tends to come off in shreds along with the lamina. The leaf should be either silent when handled, or sound like thick vinyl. If it rustles, it's too dry.
As for direction (butt to tip vs. tip to butt), it depends on the angle of the secondary veins to the stem. With a 45 degree angle, tip to butt usually tears most cleanly. With a 90 degree angle, like CT Shade or FL Sumatra, it seems to tear more cleanly from butt to tip. If the stem is too moist, then butt to tip will prevent the stem from delaminating. Each secondary vein forms a separate strand along the stem all the way to the butt of the stem. With a damp stem, this strand simply strips away from the stem.
Since any humidified batch of leaf will contain leaves of varying humidification, sometimes one direction works better than the other with any particular leaf. I usually start tip to butt. If that tends to be troublesome, I'll try the other direction.
I've actually timed the hand process, which varies with the size of the leaf. Generally, it takes 20 to 30 seconds to strip both sides of the leaf, while leaving little to no lamina fragments on the stem. I've only been able to achieve this speed by any knife method if the leaf is absolutely flat, and the stem has no twist. Doing it by hand, it doesn't matter much if the leaf is not flat or the stem is twisted.
Bob