Purposes of the transfer ports
These passages transport fresh fuel/air mixture supplied by the intake from the crankcase to the area of the cylinder currently above the piston. The transfer ports also play a vital role in cooling the cylinder and scavenging gases.
Transfer ports and loop scavenging
The fresh mixture that the transfer ports supply comes out of them at an angle determined by their casting. This angle is very precisely calculated so that instead of mixture coming from the intake and going directly out the already open exhaust port, the intake charge is blasted by the charge from the transfer ports. Instead of going directly out the exhaust, this combined charge proceeds along the upper part of the cylinder, curving back around when it hits the cylinder head, and THEN proceeding towards the exhaust. This motion is called loop scavenging. As the fresh mixture charge does this, it absorbs heat from the cylinder wall and cylinder head, aiding in cooling both. Additionally, it pushes spent fuel-air mixture out of the exhaust pipe instead of simply allowing the fresh mixture to become diluted with exhaust gases.
As the piston travels downward following ignition, it pushes fresh mixture that has moved into the crankcase upwards through the transfer ports. When the piston, on its downward travel, uncovers the transfer ports windows in the cylinder, this mixture affects the intake charge as previously described. After the piston reaches BDC, it begins to move upward again. When the bottom of the piston skirt uncovers the intake port window on the piston's upward travel, intake is pulled into the crankcase, to be pushed into the cylinder when the piston begins moving downward once more. Some engines have intake ports located on the engine cases instead of on the cylinder. These intakes are controlled by the the movement of the crank lobes. The other type of intake found on two-stroke engines is called rotary disc intake.
Transfer ports and performance
The size of transfer ports is very important in two-stroke performance. The larger the transfer ports are, the more mixture can be moved through them. At high RPMs, the capacity to move large amounts of mixture is very desirable. For this reason, it is important to match the portion of the transfer ports in the cylinder to the portion cut out in the crank case. This especially becomes an issue when adding a kit because kits often have much larger transfers than stock cylinders.