Originally designed by the Terran Hegemony for use with AgroMechs, the Retractable Blade was implemented by the Word of Blake for their Celestial series OmniMechs. During the Jihad, the technology was dispersed throughout the Inner Sphere. 
The Retractable Blade weighs one ton per every twenty tons the 'Mech weighs, rounded up, plus 0.5 tons. The blade takes up a number of critical spots equal to one per every twenty tons the 'Mech weighs, rounded up, plus one. The blade does one point of damage for every ten tons the 'Mech weighs, rounded up.