Can someone describe what Gud Asik is and give examples in Halacha to illustrate it?
Gud Asik (Lit: the wall goes up) is a halacha lemoshe misinai. It considers a pre-established "wall" to continue upwards infinitely. The wall must be a minimum of 10 tefachim from the ground and within 3 tefachim from the groundfor this halacha to apply. It is used for the good and the bad:
Example A: If a mekom petur- say a 6 foot clothesline pole- was in a backyard surrounded by a 4 ft. fence. Without the halacha of Gud Asik, if I through a towel from a reshus harabim to the top of the pole, I would be patur from a korban since it never landed in the reshus hayachid - its above the walls. With Gud Asik, the walls go up infinitely and the pole top is within the reshus hayachid making me chayav a korban (S.A. O.C. 345:5).
Example B: On Sukkos, the mechitza of a sukka needs to reach the sechach. Most practically, people put the sechach within 3 tephachim and use a different halacha called lavud to halachically eliminate the space. With gud asik you can use, say, gazebo walls that are 10 tefachim high and through the sechach on a frame 15 feet high and if the imaginary line drawn upward from the walls hits the edge of the sechach (or w/i 3 tefachim), your walls halachicaly reach your sechach. (O.C. 630:9)
When there is a collection of water that is unfit for ritual immersion of a person and some water is added to it, but not enough to make the collection fit for ritual immersion, it is possible to connect another collection of water, which is fit for ritual immersion - by means of a contiguous volume of water - above it and consider the water in the bottom one as part of the water in the top one and therefore fit for ritual immersion. The upward connection of the two bodies of water is called gud asik. (Mikva'os 3:2)