There are many applications, depending on what part of the single whip sequence one looks at.
There is hooking and pull-down and push with the right hand at the beginning of the motion.
Then there is a potential albeit well hidden elbow strike when turning around.
Then there is a combination of a ward-off, pull-down and push, the end of which can be seen on the picture. These are just the "text book" applications the way I've learned the form. With enough practice and proper mind intent, the movements may reveal more subtler applications.
The important thing to realize here is that applications in the form are given only as a guide to understanding the intent of the movements in the form while learning it. The true work and insight will come from adding to the form everything one learns from push-hands exercises, swordplay and sanshou practices, to name a few.