The grappling defense is a conditioned response against any grappling, tackle, or takedown attempt. It relies on using the bone structure of the upper-arm to stop your opponent from successfully putting you in a clinch or taking you down. The key is to step back with the leg on the same side the opponent's head is coming in on, and to use that elbow to stop his entry. Your upper arm should be perpendicular to your opponent. This technique relies on bone structure, and not muscle. Even for an opponent much bigger and stronger than you, it's nearly impossible to get in, remove, or pop up your arm from this position while entering.
To train the grappling defense have your opponent attempt to get into the clinch, tackle you, or do a single or double leg takedown. Train it both from a natural standing position and while striking.
In the images below you see one counter after the takedown attempt is stopped. We normally prefer not to punch to the head, but in this case the jaw is completely exposed and open for a perfect hook. Many other follow ups are possible.
See more self defense techniques, or more from the Fundamental Five: