This is the conventional method of slicing food. Hold the food firmly on the chopping board with one hand and slice straight down into very thin slices. If you use a cleaver rather than a knife for this, hold the cleaver with your index finger over the far side of the top of the cleaver and your thumb on the side nearest you to guide the cutting edge firmly. Hold the food with your other hand, turning your fingers under for safety. Your knuckles should act as a guide for the blade.
There is a technique for splitting food into two thinner pieces while retaining its overall shape. It can be used for cutting chicken breasts in half, for example. The cleaver with its wide blade is particularly suitable for this. Hold the blade of the cleaver or knife parallel to the chopping board. Place your free hand on top of the piece of food to keep it steady. Using a gentle cutting motion slice horizontally through the food.