There are many different approaches/styles to guard passing. Most of the time, they fall into the three following categories:
1) Over the guard
2) Under the guard
3) Around the guard
Alternating between these three types of passes makes it hard for the bottom person to stop, and is what most high level players do.
Guard and passing the guard can be viewed as an arms race, where new types of guards/moves are developed and new passes are also developed to counter them. A particular guard may be popular because a lot of people are not yet familiar with how to deal with it (the 50/50 for example) but as time goes on, the focus may shift to a different position. Also as a response, the corresponding guard passes also shift in popularity.
Now a great classic between Reuben Corbinha Charles and Ryan Hall