There are logic rules in place which prevent you from being locked out of progression and guaranteeing that you’ll be able to finish the game every time regardless of the distribution of items. The different game modes use different logic and assume different levels of game knowledge and ability.
This mode requires no advanced knowledge about the game. It’s designed as if you were playing the original game for the first time.
The logic will prevent you from getting stuck anywhere regardless of how you use small keys within dungeons. However one small caveat - you will never be required to use a small key found in the second part of Desert Palace to unlock the door in the first part. Doing this could lead to a softlock.
This mode may require knowledge of various techniques and minor glitches.
In addition there are some other minor changes from Casual mode.
This mode accounts for everything besides EG and semi-EG which are two extremely powerful glitches allowing you to walk through floors or under walls… and even reach the Triforce in under 2 minutes! This mode is very difficult and may require advanced knowledge of some other major glitches in addition to everything in Speedrunner mode.
These techniques allow you reach a multitude of locations with very few items. To learn more please see this guide
In addition some of the changes listed in the above section have been removed to allow this version to work correctly.