Mountain bike shoes
Toe cages have been the standard in for pedal-optimization for many
years; by securing the foot to the pedal with a cage, there is less
energy wasted in the upward portion of the cycle. Now, clipless mountain
bike shoes are replacing toe cages because they drastically increase
pedal-efficiency.
Clipless mountain bike shoes require a corresponding pedal to hook
onto; once connected, a rider will not be able to lift their foot off
the pedal (that energy goes to lifting the pedal instead). Rather, the
foot is removed be twisting out of the pedal–which can take some
getting used to.
Energy-conservation is the main point of clipless mountain bike shoes–the
idea is that a rider should be utilize as much force in the upward portion
as the downward portion of the cycle. Even toe cages allow some movement
(up/down, side to side) that could be used to power the bike.