Choice quote:

"Not One Step Back" outlined some of the practical measures that would be introduced. Any soldiers of whatever rank found guilty of a disciplinary offence would be sent to penal battalion (known as a shtrafbat). These units would be used for marching across, for example, a minefield before a main infantry unit moved up. Order 227 stated that men in these units had to atone for their crimes against the Motherland with their blood. Another new unit created by ‘Not One Step Back’ was the ‘blocking battalion’ (known as the zagradbat). Men in these units would be at the rear of an attack and would shoot anyone not advancing quickly enough or were seen retreating. Blocking battalions would end up shooting thousands of Russian soldiers at the Battle of Stalingrad for failing to show the necessary resolve in combat.

some links:

http://www.mishalov.com/Stalin_28July42.html

http://www.historylearningsite.co.uk/not_one_step_back.htm

http://www.learnenglish.org.uk/militaryenglish/magazine/people.asp?person=70