The modern-day swing is all about bat speed (and launch angles!), borne out of strong hitters and maximum weight transfer. Today’s hitters generally start with some type of load mechanism, with their weight shifting back. Then, in a sudden shift of power, everything goes forward in violent yet controlled fury.
A hitter’s back leg is there to support that initial load and to provide something to pivot against, but by the time a hitter makes contact, it’s really just there for the ride. Here’s Mike Trout‘s first career home run (from hitting analyst Ryan Parker):