asterisk users - Jan 2011 - Why Local Channels are creating

Hi
     Does anyone know why Local Channels are creating in asterisk 
(1.6.1.1)?E.g. If I do call forward  4 channels and two threads are 
creating,it will delete after the call disconnected . In the 4 channel 2 
of them of then are  SIP channels and 2 of them are Local channel.Pleas 
tel me why this is using..?
Thanks
Nikhil