There are two types of pruning secateurs: bypass and anvil. This is an explanation of the pros and cons of each type, when you should be using them and why.
At first glance there does not seem to be much of a difference between these tools, but the difference is significant.
Bypass secateurs work like scissors with two blades passing each other closely.
In most cases you will be pruning live plant material or wood that has recently died. When you use a bypass secateur correctly (see below), you will do almost no damage to the plant stems or branches that are left.
Anvil secateurs work like a knife where a blade is pushed through the plant material onto a cutting board, i.e., the anvil.
Anvil secateurs tend to crush soft plant tissue and are not recommended for general pruning. However, anvil secateurs work better than bypass secateurs for cutting old dead wood, particularly where tough fibrous stems may distort bypass secateurs.
How to Use Secateurs
The secateurs should be held so that only the blade touches the wood or stem that will be left on the plant. This means that you might need to turn your hand depending on your relative position to the bud, or to move around to the other side of the plant. If you follow this rule, you will cause little damage to the plant.
To make a cut close to the bud/node, the secateurs should be held very close to the bud. With anvil secateurs there is a risk that the bud or the tissue around the bud sits on the anvil and gets damaged during the cut. Using bypass secateurs, you can hold the secateur so that none of it touches the bud during the cutting process.
Create clean cuts that slope to run rainwater off the cut and to the outside of the plant. The ideal slope of a cut on a vertical stem would be 20-30°.
The diagram below shows the proper way to hold the secateur to cut off a branch from the trunk.