Ladybird popping........what is it?

I filmed this rather large ladybird on our pond yesterday..........

When I'd downloaded the footage I found that it did a large 'POP' when it threw out its back leg-wing type thing. 

Any insect people out there know what it was actually doing and why? 
If you know please leave a message on my Youtube page.

Here's the Youtube vid, plse click here to view:

Ladybird popping



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