Rainbow body (Tib. འཇའ་ལུས་, ja lü; Wyl. 'ja' lus) — fully
accomplished Dzogchen practitioners can dissolve their body at the
time of death.
Through the practice of trekchö, the practitioner can attain the
so-called ‘rainbow body’, in which the body becomes smaller and
smaller as it dissolves, emanating rainbow light, and finally only
the hair and nails are left behind.
Through the practice of tögal, the practitioner can dissolve his or
her body into the ‘Light Body’ (Tib. འོད་སྐུ་, ö ku), where the
body transforms into light and disappears completely into space.
This was done by Garab Dorje, Manjushrimitra, Shri Singha,
Jnanasutra and Vairotsana.
Another accomplishment of tögal practice is the ‘Rainbow Body of
Great Transference’ (Tib. འཇའ་ལུས་འཕོ་བ་ཆེན་པོ་, ja lü phowa
chenpo; Wyl. 'ja lus 'pho ba chen po), where the master dissolves
his body into rainbow light and lives for centuries in order to
benefit others. Such was the case with Padmasambhava, Vimalamitra,
Nyang Tingdzin Zangpo and Chetsün Senge Wangchuk.
http://www.rigpawiki.org/index.php?title=Rainbow_body