A cab or caballerial is a 360 fakie ollie, so a halfcab is a 180 fakie ollie.
If you can do regular 180’s, you won’t have many problems learning halfcabs.
1. Start riding fakie with your toes on the toeside of the tail.
2. Turn your shoulders in the direction opposite to your jump direction.
Crouch down a little and get ready to jump.
3. Pop the board and jump. If you pop on the toeside of the tail, the board will automatically turn 180 degrees.
Swing your arms to help you make the 180 rotation.
4. Use your front foot to guide the board.
5. Land and roll away.