Heel struts

"Heel struts" (also sometimes referred to as High Heels) are part of transformer anatomy. They commonly appear on Transformers with a relatively low surface area on their feet, which has been concentrated somewhere forward of the ankle joint.

This arrangement allows a Transformer greater dynamic stability by re-locating the ankle joints away from the center-of-balance axis, and reduces wear on the ankles by allowing the foot to flex before absorbing an impact. Heel struts can increase the leverage afforded by the ankle joints by distributing the pressure over a larger area, dramatically increasing a Transformer's static stability without the need for big, clunky feet.

Heel struts are more common on Transformers with small frames or a high center of gravity. It has been noted that a disproportionate number of female Transformers have heel struts... but female Transformers also tend to have smaller frames and lack the big clunk feet of many male Transformers.

Also, they make their butts move sexy when they walk.

Heel struts should not be confused with reverse articulated knees or triple-jointed legs, both of which feature the forward-canted foot, but typically lack the stabilizing strut behind it.