I'm not sure when I first heard the term "impeachment nugget," but it changed the way I think about depositions.
An impeachment nugget is a question and answer in a deposition about a single fact, which can be easily used at trial to impeach a witness who tries to change his story. A very simple example:
Q. The light was green, correct?
If you think in terms of impeachment nuggets as you're taking a deposition, it will help you maintain a clean transcript whenever you're doing the important work of pinning the witness down about key facts. You'll remember, for example, to confine your question to a single, simple fact. You'll also remember to keep repeating the question until the witness gives you a straight answer without extraneous information.
In the example above, if the witness answered, "Yes, and I thought it was about to turn yellow," it would be appropriate to say, "I'm going to ask again. The light was green, correct?" When the witness responds "yes," then stops, you have an impeachment nugget for trial.