"Shtrafbat" (Punishment or Penal Battalions) were made up of deserters, political prisoners, and former POWs who had returned from German captivity (according to Stalin: allowing ones-self to be captured alive & imprisoned was akin to surrendering, & surrendering was considered to be treason, a crime punishable by death), and even "regular" criminals sent from prison-camps, so the suspicion that these people wouldn't fight well, or might even surrender, was high. These "criminals" were "graciously" allowed "a second chance" by the government, a chance to "wash away their past sins with their own blood."
These "Shtrafbat" were sent on the most dangerous and difficult missions (i.e. those with low survivability) and were followed into battle by troops who were called "Zagrad-Otryad" (Blocking Troops), and were under NKVD (precursor to the KGB) control. Sometimes these troops were almost as large as the actually fighting units they blocked. They were under orders to shoot any Soviet soldiers who were thought to be retreating, be they wounded or not.
The few "Shtrafbat" soldiers who lived through it for some months, were considered rehabilitated and returned to regular army units. The KIAs were rehabilitated posthumously, sparing their families from being branded as families of traitors, "enemies of the Soviet people." - IMDB