All defenders must tackle. It is important to have good technique to
prevent injury. Most forms of organized football have strict rules
in place to prevent cheap tackles likely to result in injury. Aiming
contact for the head or below the knees is forbidden.
In the early days of organized football, when players wore light pads, tackles were similar to rugby. They focused on dragging a player down. With improved pad technology, players have moved to stronger hits aimed and leveling their recipients. It can be argued that improved pads have worsened player's health.
Proper technique is to lead into the tackle with the shoulder and wrap your arms around the ball carrier. Professionals are often seen leaping into the air to deliver massive blows, although this isn't advisable.
Unfortunately, full contact sports take their toll on the body. Often, players are visibly affected by the hits they take. Permanent injuries have occurred.