Instructors see it
day in and day out: skiers struggling with worn-out, poorly chosen or ill-fitting boots. The challenge for the teacher is enormous, but it's even greater for the student.
Boots are a skier's most important equipment investment. Selecting and fitting the right pair takes time, research and, ideally, expert assistance. You might need to return to your bootfitter several times for small adjustments. But once you're dialed in, you can stick with a pair of boots for several years.
Boots that fit properly are efficient conduits of both information and energy. If your boot is too loose, your foot can turn within it (A), while transmitting little or no turning force to the ski. At best, there may be a delayed reaction on the part of the ski, compromising control. In fact, if your boot is so loose that your foot won't stay flat on its sole, your ski can overpower your leg: The ski will actually flatten when you try to edge it.
On the other hand, a boot is not a cast. Boots that are too tight constrict foot and ankle movement, as well as circulation (one reason your feet may get cold). Boots that are too tight will also hurt, which is not only unpleasant, but a performance problem as well since pain hampers foot sensitivity, making it tough to discern what your ski is doing on the snow.