Yevamos 33

The Gemara gives the case that a Kohain becomes Tamia and becomes blemished at the same time is when he cuts his finger off with a Tamai knife. Tosfos deals with the problem that both doesn't happen at the same time. The knife makes him Tamai at contact and he's not blemish until he cuts off the whole finger. So Tosfos explains that we refer to a case where the finger was already cut and was hanging on by the smallest amount. Therefore the contact of the knife also immediately severs the finger, rendering him a blemished Kohain. Rabbi Chaim Smulowitz If you ever though that Tosfos was beyond you. Think again. A few more free copies of Gemara and Tosfos: Sukka 2a-11a is still available at https://www.tosfos.ecwid.com