The user entering symbols is required to wait for a brief period, for example 500 milliseconds, between each symbol that they write. This delay allows the system to tell when the user has finished each symbol.
This is appealing because it is easy to implement, and offers a high accuracy of determining where each symbol ends. Unfortunately, it is frustrating for a user to write like this as it is not natural. Being forced to concentrate on writing slowly also takes away from the user's ability to concentrate on the formula itself.