From my personal experiences tickling my fiancee, I feel like when she's watching me tickle her, she can anticipate it and prepare herself, as opposed to me blindfolded her and surprising her with little tickle attacks at various spots along her body. The anticipation and the surprise heightens the senses. When you're expecting something it can go either way. I won't sit here and tell you it makes you less ticklish because for some people, knowing what's about to happen makes it worse for them. But for other people, it allows them to prepare for it and resist.
What is very fun though is telling your lee what's about to happen or talking about it. I enjoy playing this game with my fiancee where she tries not to laugh and she normally is pretty good, especially if thr TV is on.
However, when I start asking if she can feel this or how does this feel or saying things like "I bet you're trying not to focus on my finger lightly tickling the center of your arch", it really fucks with her head and her concentration.
She had a steadfast defense once and I was about to give up until i started describing to her exactly what I was going to do to her and I was telling her how bad it tickled her and how crazy she must be going, and sure enough within seconds there was increased wiggling, whimpering, and eventually full blown laughter and begging.
Your lee doesn't always have to watch, but if she's trying to focus, trying not to think about it, MAKE SURE they're thinking about it.