Vedas and Upanishads (especially Prashna and Aithireya) mentions about Shukla Patha (pure or bright path) and Krishna Patha (dark path). Shukla patha leads to Indra yoni (gat)e at Solar disc (vaikunta dwaram), through which Vaikunta Loka could be reached. There is a special Naadi (channel) from the base of soft palate in the mouth from the center of Uvula going upto Brahmarandra (at vortex of skull). From this Brahma randra, the shukla patha starts. All these paths and channels are at subtle (shukshma) level and are not visible anatomically. Whereas the Krishna patha (dark path) leads to Hiranya Gharba and this path can be reached by any channel in the body.

So when a Gnyani dies, his Jiva (soul) leaves his body through this subtle channel from Uvula to Brahmarandra, which opens automatically into shukla patha. His Soul passes through Indra yoni at solar disc and enters Vaikunta. From there gnyani’s soul goes to Brahma loka and gets the final Brahma gnanam from Brahmaji himself and attains moksha.

When an Agnyani or a common man dies, his soul would leave his body from any one of the channels of the body and enters Krishna patha and goes to Hiranyagharbha only to return back to next birth, depending on his prarabda karma. 
All the above channels mentioned are at subtle levels and nobody can open it manually. Most popular Vaikunta dwara opening on Vaikunta ekadashi is based on this vedantic / philosophical basis