Shellshock vulnerability, CVE-2014-7169

Question

This ycombinator thread states that

env X='() { (a)=>\' sh -c "echo date"

allows you to drop the first token, and then run the result, sending the output to the dropped token. I can see that the magic is in the () { (a)=>\' part, but this is quite a hard thing to search for, so it would be great if someone could explain the inner workings.

In the linked possible duplicate question, a variable is exported to an environment variable by using,

env X="() { :;} ;

which really is not the same as

env X='{ (a)=>\'

which is the part I am trying to understand the workings off. So, what do the (a) and =>\' parts do? Clearly, it is doing something very similar, but is different in that it got around the first patch to this vulnerability, see comments by chepner.