The gemara discusses whether somewhat watching fruit in a field is obligated to dwell in sukah. He could build a sukah near his field. The gemara says he is exempt. Why? Abaye- need "ki-ein taduru" and there isn't because he couldn't schlep his stuff out to field. Rava-If he would be inside sukah then robber would go on non-entrance side and guard wouldn't see him. "Mai beineihu?" If the fruit is in a pile such that he could watch it even inside of sukah. According to Rava he would be chayav in sukah, according to Abaye no. What I didn't understand is Tosfot who say that Rava is adding a reason but also agrees with Abaye and accepts principle of "ki-ein taduru". If so then there the example the gemara gives is not a difference between them. Both would exempt guard because of "ki-ein taduru".
Tosfos does not mean that Rava agrees that in this case the exemption of "k'Ein Taduru" applies. He agrees that "k'Ein Taduru" exempts a person in other situations; here, though, he argues and says that "k'Ein Taduru" does not apply in this case. (For a better understanding of the Machlokes between Rava and Abaye, see our Insights to that Daf.)
M. Kornfeld