WiseGEEK could tell you exactly what an aposiopesis is, but... I'm sorry, I just can't go on. Can we talk about some other rhetorical device instead?
The above snippet is an example of a rhetorical device known as an aposiopesis, from the Greek for "becoming silent." An aposiopesis can appear in different forms, but generally speaking it is a deliberate interruption or pause used to create dramatic tension. An aposiopesis is often created through the use of a dash (-) or ellipsis (...) to imply an idea which cannot be or does not necessarily need to be finished by the speaker.
If a mother says to her child "Put down that toy right now or else I'll...", she has used a form of aposiopesis to imply a threat of punishment. The speaker set up a condition, but the listener did not need to actually hear the rest of the sentence to understand the consequences. An aposiopesis is most effective when the listener(s) can successfully deduce what is missing.
Another form of aposiopesis occurs whenever the speaker becomes too overwhelmed or distracted to finish a complete thought. At a funeral ceremony, for example, a speaker delivering a eulogy may need to pause in mid-sentence to collect his or her thoughts: "I'll always remember my best friend Ray... I'm sorry, I can't find the words... Ray was just...". These pauses indicated by ellipses would be considered aposiopeses. When a speaker becomes unable to continue, the resulting pause would be an aposiopesis.
Many playwrights and screenwriters use aposiopesis to make dialogue sound more realistic or sincere. Many famous soliloquies in Shakespeare's plays are meant to be delivered with emotional pauses and moments of silence, not as straight recitations. A character may also use an aposiopesis to create a dramatic or comic tension in the scene, as in a pivotal moment when a detective is about to name the real killer: "I finally put the last pieces together and I know the killer is-- Are those flowers real? I can't ever get anything to grow in my office. Anywhere, where was I? Oh yeah, the real killer...".
An aposiopesis can be a very effective rhetorical device when used sparingly and under the right conditions. If the listener cannot possibly deduce the desired meaning from the speaker's opening premise, then an aposiopesis may not work as well. Many aposiopeses are presented as conditional cause and effect sentences, with the speaker only providing the cause or premise: "If I have to come up there...", or "If only I had a tape recorder right now...". The listener should be able to provide a logical conclusion of his or her own based on the speaker's deliberate pause.