Take a look at these cases which are reed cases.
http://www.davesmotors.com/s.nl/c.88...t.A/id.1308/.f
If you notice the induction charge point (air fuel mixture) is going into the case instead of the cylinder like on most engines and where the induction charge goes in uses a reed valve to allow the charge to go in or to close it off at the correct time. This is done by the vacuum of the piston going up and down in the cylinder.
On most of the engines we run uses a piston port system to induct the charge. When the piston is down the port is open when the piston goes up it closes the port by way of the piston skirt.