Very interesting thread!
The allure of bondage is, of course, that your ticklee can't escape. But there may be something else to it.
When I was growing up, most of the tickling I did was just a quick poke in the side of some girl in my class. Back then, that was as much of a thrill as I could ever feel. Even more extreme, though, was the less common occasion when I could tickle a girl for a little longer -- say, grab her ankle and tickle her foot for the few seconds it took to pull away. Jump to a few years later, I'm thirteen or fourteen, and now strong enough to hold a girl down and tickle her for thirty seconds or a minute. See where this is going? The fascination, for me, became the idea of tickling for longer and longer periods of time. To fantasize about tickling a girl who was tied up was a natural thing. I was so drawn to the idea...I wanted to know: what would happen if you tickled a girl, and just didn't stop? Just keep tickling and tickling...ok, ok, you've all been there. At least many many of you have.
The funny thing is (actually not so funny; it makes sense) is that now that I've done some tickling with bondage, and seen pictures and videos of it, I've come back around to enjoying more playful tickles in which my ticklee has at least a chance to escape. I've got the advantage, of course; I'm not the biggest guy (5'10", 145 lbs.) but I'm wiry and strong. And when you tickle a girl who's not tied up, you have to use your entire body to hold her down -- which does require more effort, as Blackstar pointed out, but there's something sexy about wrestling that adds to the thrill.
It's nice to have choices in life. Sometimes you want a good, all-out, rolling-around-all-over-the-place tickle, so you leave the ties on the rack. But to tie a girl down and have total control over how much she gets tickled, and for how long, is an absolutely insane thrill I highly recommend ;-)
