This term denotes a common experience with  sudden contractions of the
big body muscles while falling asleep. This mostly causes a feeling of
stumbling,  falling or similar and subsequently  waking up again.  The
exact  cause  is not  known, it probably  is   some disturbance in the
brain's functions  in the first  stage of sleep.  Surely it is common,
and does not cause serious problems unless it  stops you from sleeping
(but then you have general sleeping problems as well).

[From: Corey Thompson <COREYT@vax1.mankato.msus.edu>]
In  my Psychology  class,  the   professor  said  something  to  like:
"...you're  heart rate gets very  slow,  and and  your breathing slows
down quicker than  normal. You brain  may interpret this as your  body
dying,  so it sends  an electrical pulse to your  muscles. Like a jump
start. This is similar to a Night Terror, when  you wake up absolutely
terrified about something, but have no idea why...."