Depends on your reason for recovery.
If it is because of corrupted configs, lost password, or corrupted update installation.
Then during reboot you will see a GRUB Menu. Select the 2nd option to boot the Recovery Partition. Booting the Recovery Partition will reformat the main Application Partition and install a fresh set of software. All of the configuration will be cleared except the IP Address, Netmask, Gateway and a few other management interface paramters, and it will set the password back to the default "cisco".
If you are remote so can not access the sensor console, but you do have CLI access to the sensor.
Then you could alternatively execute the "recover application-partition" command under the "conf t" mode. It will change the default boot partition to the Recovery Partition, reboot the box to the recovery partition and do the same as above.
I would thne give it about 20 minutes to complete.
To once again access the sensor you will need to use SSH to connect to the sensor and login with the default cisco username and cisco password.
If you need to re-image because you believe someone has tampered with the sensor. Then you will need to order a recovery CD. They may have tampered with both the application partition and recovery partition, and a CD would be needed to reformat the entire drive and load both partitions form scratch.