The River Clip is used to enter Hyrule Castle or Agahnim's Tower with the help of Major Glitches

With Boots and the Hookshot

Align the top of Link's head with the top of the ledge located to the south east
Do an Item Dash with the Hookshot ( Y + A ) and move to the right while holding B
If you can not move any further to the right, hold A and you automatically drop down the ledge
Dash 1 screen to the right, move a bit down, and continue dashing to the right
You will enter the screen west of Hyrule Castle and can now enter Hyrule Castle or Agahnim's Tower

Tips and Tricks

If you bonk while having EG, you have to reset your console to get back to the state. Depending on the category, the run is over
If you are fast enough, you do not need to kill the Green Guardian
If you clip to low, you will end up on the wrong position. The outcome is different, so you will notice