As noted in
Pern 101 at http://www.randomhouse.com/delrey/pern/pern/pern101.html.
The strange, cold, black area of nothingness dragons go through when they teleport.
The dragons of Pern teleport for a variety of reasons. This teleportation is not instantaneous and the time taken is spent between. Dragons can also carry riders when they teleport and the riders find going between an unpleasant experience. Rarely a dragon will go between and not come back.