Cicero once told a story about King Dionysus forcing a fool named Damocles to sit in his throne with a sword suspended above him by a single hair. The (literal) suspense was so great that Damocles finally begged to be relieved of the throne. The "Sword of Damocles" has come to be a warning about the constant fear and anxiety that hangs over people that assume a position of power.
In aikido, shomenate is the Sword of Damocles. Tori would much rather end up behind uke but in this first technique, tori makes a critical mistake and steps to a position directly in front of uke almost toe-to-toe and between uke's arms! Tori has given uke the Holy Grail so far as positional power goes, but along with that power comes the threat of shomenate.
In Southern U.S. (redneck) culture we call this concept "The Hammer" instead of The Sword of Damocles. The Hammer is your one great backup plan or technique that you are holding in reserve in case the rest of your plans go awry. As tori you would prefer to step behind uke, but every so often you can't or you just don't so you end up toe-to-toe and between uke's arms - so you let The Hammer drop - and more often than not, shomenate will extract you from this terrible position.
We often talk about how all of our aikido system is built around shomenate and the consequences of shomenate. That is true, but another way of thinking about it is that shomenate is such a great backup plan that it gives us the psychological freedom to explore the rest of the aikido system. Since we know that we have a good chance of extracting ourselves from just about the worst mistake we can make by using shomenate, we can safely relax and work on the other techniques.
Shomenate is tori's Hammer and uke's Sword of Damocles.