F9->Shift-F9 Exception passing in Olly

[bgrimm]

Perhaps a simple "How does it work that way?" question:

I was just practicing on an ExeSheild packed target and was wondering how/why Ollydbg acts differently when, after triggering a memory access violation, if you hit "F9" THEN "Shift-F9", it stops program execution, just "Shift-F9", proggy runs off on its own.

If that didn't make much sense, I am confused on WHY this works:

For a simple unpack of an Exeshield target, I just loaded it in Olly.
Then ran it and it hit a memory access violation:

00405B52 8B00 mov eax,dword ptr ds:[eax]
00405B54 EB 01 jmp short TestApp_.00405B57

Access Violation when reading [00000000] ... blah blah blah.

Now if I hit "F9" and then pass the exception with Shift-F9, I stop one byte past the OEP and all is good.
If I just pass the exception with "Shift-F9", proggy runs and no stop... ok, whatever.

When I'm stopped at the exception and I press "F9", nothing really seems to happen, its not appearing to trace an instruction or anything, so what does it do? <confused>

A tutorial from Hacnho had this F9-Shift-F9 steps in it but it did not really explain WHAT had happened.

Thanks,
-bg