The 1975 Taekwondo poomse textbook describes Sipjin as
follows:
"The decimal system is the standard numerical value of
hundred, thousand, million, billion and so on. In this sense 10 is the
symbolic figure which means endless development and growth. but the growth
is always affected by systematic and orderly rule. The life of Sipjin lies
in the supreme change and orderly discipline of the decimal system. Stability
is sought in every change of movement."
The new Kukkiwon Textbook describes Sipjin as follows
(slightly different from the older explanation):
"The word 'sipjin' was derived from the thought of 10
longevity, which advocates there are ten creatures of long life, namely
sun, moon, mountain, water, stone, pine tree, herb of eternal youth, tortoise,
deer and crane. . . . Sipjin symbolizes these things. . . . The Chinese
letter meaning ten is the form of the poomsae line, which signifies an
infinite numbering of the decimal system and ceaseless development."
Sipjin is the poomse for 5th Dans and the form symbolizes the exponential growth that many 5th Dans experience. After sacrificing his own development for the sake of the development of his students and his own teaching style as a 4th Dan, for some reason, the 5th Dan undergoes a magical transformation, and one of the telltale signs is the often ten fold increase in knowledge the accompanies this period of development. One new concept gets turns into ten new ideas or techniques, and so forth. However, one must concentrate and remain grounded during this period of extreme growth and expansion, otherwise there could be negative ramifications on the practitioner's future progress, and Sipjin's emphasis on stability is a reflection of that.
The USTU has many sad stories of practitioners in this phase of development who got carried away with their own development and suffered the consequences. Had they heeded the lessons inherent in the poomse Sipjin, perhaps some of these situations and consequences could have been avoided.
The changes which occur during Sipjin, like Pyongwon, are mostly mental in nature, as opposed to physical like in the earlier dan levels. The 5th Dan is a very important and very exciting time, but the experience will be directly proportional to the foundation that was built before it, during the lower dan levels. If one skips or failed to fully learn the lessons at the lower level, then there will be no exponential growth and no tenfold increase in knowledge, ability and awareness. Once the 5th Dan fully experiences this exponential growth spurt, then he or she is ready for promotion to 6th Dan and the lessons to be learned as embodied in the poomse Jitae.
Special thanks to Master Glenn of Hawaii.