The clutch slave cylinder should be completely compressed during installation so excess air is not allowed into the hydraulic system. Many replacement clutch slave cylinders come with a plastic band or cap to hold the rod in place and keep the cylinder compressed.
The plastic retention device should be left intact during installation. After installation of the clutch slave cylinder, the system must be bled. This is done by depressing the clutch pedal, which engages the master clutch cylinder and forces fresh hydraulic fluid into the new clutch slave cylinder. As the clutch slave cylinder fills with fluid, the plastic retention device snaps, allowing the rod and piston to extend fully.