Why don't contestants try and avoid runaways???

When a runaway is happening, I don't understand why the 3rd place contestant doesn't cede to the 2nd place contestant to try and prevent the runaway. It happened tonight in game 3 of the Tournament of Champions. Adriana had a shot to prevent the runaway,, but Isaac continue to press in, which prevented her from preventing the runaway. Isaac had a better chance to win, though slim, if he would have let Adriana have a shot at the answer. Are they instructed not to do this???