Trapped players have a birthdate that is in November or December. Because their birthday is beyond the school start cutoff date, which is usually November 1st (it used to be January 1st) they are not allowed to start school "early"; they would be 4 years old in Kindergarten for about half the year, then turn 5 like the majority of their classmates for the remaining half of the school year. Because the US Soccer year group cutoff date was changed to Jan 1st, and not Nov 1st, they are forced to play with a group of players that is in a Grade Year ahead of them, and forced to attended school a grade year behind the Calendar Year Group of their soccer teammates. This is why they term "Trapped Player" was created; they are literally caught in the red tape between two inflexible rules based on arbitrary dates.
You are right about how it works. Just want to clarify that these players would not start Kindergarten when they are 6. They would be 5 years old for about half the school year. My daughter is in this trapped player group, which I'm sure everyone already has figured out by my over involvement in this discussion. When she started Kindergarten (the first time) she was 4 years old, about two months into the school year, California changed the law regarding the cut-off date. The school gave us an option to disenroll her and come back next year, or keep her in the class under a "TK" enrollment. Either way she would have to attend Kindergarten the following year. Because she was already about two months in and enjoyed it, we decided to just keep her in school. Six of her classmates where in the same scenario. They all did Kindergarten twice; the school board refused to let them move into 1st grade with their first Kindergarten class.