Actually, just "something" has to touch... here is the USMS backstroke turn rule...
Originally Posted by moodyrichardson
Upon completion of each length, some part of the swimmer must
touch the wall. During the turn the shoulders may be turned past the vertical
toward the breast, after which a continuous single arm pull or a continuous
simultaneous double arm pull may be used to initiate the turn. Once the body
has left the position on the back, any
kick or arm pull must be part of the
continuous turning action. The swimmer must have returned to a position
on the back upon leaving the wall.
The swimmer who turns past vertical and, in a continuous motion,
grabs the wall before pushing off with the feet while on the back is consid
to have executed a “continuous turning action.”
Of course, bashing the end with some parts might be uncomfortable!