When fighting Titan vs. Titan you can go into melee combat and try to punch the other titan. Sometimes (if the Titan is not auto-controlled) you can perform an execution, where your Titan will grab the enemy Pilot and throw him away. What are the requirements for this to happen? Does the other Titan have to be already doomed or are there some other factors?
The other titan has to be doomed, and you have to be in front of it. These appear to be the only requirements.
P.S. The differant chassis have different executions. Atlas - Throw. Stryder - Squeeze. Ogre - Beaten to death with the it's own arms.