"this is the kicker"

Why do people say "and this is the kicker", prior
to a point they want to emphasize?

Well, being kicked gets people's attention.

That's true. But it still seems strange to me.

The only other thing I've found searching around
on the web is that a "kicker" is a side card in
poker that is the tie-breaker in hands with the
same rank.

I found another reference, but it doesn't talk
about why:

www.bartleby.com/ 61/42/K0054200.html

"A tricky or concealed condition; a pitfall: 'The kicker is that the relationship of guide and seeker gets all mixed up with a confusing male-female attachment' (Gail Sheehy)"

We've got some card sharks on this site. They can probably come up with a good explanation. Stay tuned.