Change block tracking allow tools to track what blocks in a virtual drive have changed over a period of time. This is very useful for backup software as it is able to backup just the delta changes instead of the whole virtual server.
Shutdown your virtual guest. On the virtual guest, edit Setting -> Options -> Advanced -> Config Param
add in the following parameters:
ctkEnabled TRUE scsi0:0.ctkEnabled TRUE
NOTE: Duplicate the bottom line for each SCSI drive you have on the guest. Looks at the drive setting to figure out the SCSI bus.
Once enabled, you should start seeing ctl vmdk files getting created for each of your virtual disks.