There is no valve in the cylinder of the chainsaw, one possible explaination is that the piston ring location dowel has come out of the piston but this would really cause major problems as the gap in the ring would eventually foul on the inlet or exhaust port and break it but it is one explanation. to find out if this is the case, next time it happens whip the carb and the exhaust off and look up the ports, you should NOT see the gap in the piston ring. If this is happening then good luck - it may have destroyed your cylinder pot!
If the saw does it again, it may be worth checking your starter pulley mechanism, it usually has a couple of sprung loaded centrifugal pinnions on it that fly open under starting, these may be faulty.
Difficult one to sort without seeing it - does it turn over freely if you pull the cord slowly?
Steve, March 2009