Foot radical on the left - the indexing semantic.
跨 means to step over, straddle, cross. The
足 places the action squarely in the leg-and-foot family:
跑 (run),
跳 (jump),
踏 (tread),
跟 (follow at the heel),
越 (leap over). The radical pins
跨 as a foot-action, specifically a long stride.