The image isn't the best as it was difficult to find a good example. However, you should get the idea.Mano sensei identified three key areas when this posture is taken by the opponent - kote, tsuki and gyaku-do. Distance and height of arms will dictate which target to attack.
Tsuki - If the arms are high enough then there is an opportunity to attack tsuki. This can be followed up with ni-dan waza (e.g. tsuki-men) if the initial thrust is ineffective. If the arms are low, covering the tsuki then there is an opportunity to attack men.
Gyaku-Do - Mano sensei explained that a right sided do cut can be made if the opponents posture allows. Sensei encouraged a follow though to the left rather than hiki waza style retreating zanshin.
Hidari Kote - This is a small target to attack. If the opponent's hands are far over defending the men/Do then it leaves the bottom of the left kote open
The main problem with these waza is that they have to be performed without hesitation. If too much time is taken to decide then the opportunity will be lost.
[edited 02/03/11]: I have since found out this defensive posture is called 'Sanpomamori'. Kenshi247 writes a very good article about it's use in modern kendo here.
