See Advisory Circular AC61-65F Appendix 1

70. To act as PIC of an aircraft in solo operations when the pilot does not hold an appropriate category/class rating: § 61.31(d)(2).
I certify that (First name, MI, Last name) has received the training as required by §61.31(d)(2) to serve as a PIC in a (specific category and class of aircraft). I have determined that he/she is prepared to serve as PIC in that (make and model ) aircraft . Limitations: (optional).
/s/ [date] J. J. Jones 987654321CFI Exp. 12-31-19

The Soaring Safety Foundation and local DPE's advise instructors to insert a time limit in the optional limitations. Otherwise you are signing the "student" to act as PIC in a glider for the rest of his/her life.

The local FSDO and DPE's have accepted this endorsement for experience flights and practical tests for add on ratings without having a current flight review in another category and class.