Populate an embedded form field when the task is claimed

Hey all.
I am wanting to get the current user details and populate some fields on a task embedded form, but only when the user claims the task. I can easily get the current user details, and I have the form/variable load events, however I do not know how to populate fields ONLY when the task is claimed. Is there a listing of task list events that we can use, or is there some other way I can get this information?