In MMA, unlike in regular BJJ competition, you can punch. This makes it very difficult sometimes to execute submissions and other techniques, as you must always be protecting yourself. However, much of the time, if you are willing to take a little damage, strikes open up many submission opportunities.
A big one is when the guy in guard posts his left arm on the chest or the neck, and punches with the right hand exclusively. When he does this, the guard player has an opportunity to armbar the posting arm. As was seen by Diego Brando in the TUF final this last Saturday. Here is an animated gif of the final moments:
This is an important technique to work with, because in a street fight or in MMA competition, these opportunities will arise. Often untrained guys, and guys that do not train in gyms where submissions are heavily trained and used, will make the mistake of posting one arm and pounding away with the other.
In guard, you always need to be aware of the threat that the guy on the bottom poses. If you can punch do so by alternating arms or, be sure that you are in a safe position before dropping your bombs. Only post on the neck if you are immediately going to attempt a pass. Never just stay there in that position, as you are vulnerable to the guard player's submission attack.
Just for fun, here is another angle: