in tong bei
we have the knuckle protrusion of the index finger (pheonix eye or feng yan chui)
the knuckle protrusion of the midder finger (bone penetrating or tou gu chui)
they are designed to deliver focused strike on soft spots of the opponent
We practice on a pad or flour bag on a table or hanging bag
but one has to be very careful about the alignment of the wrist and elbow to shoulder
not done right
wrist hurts bad
or shoulder hurts bad
etc etc
in actual use, they are to deliver only a small or focused enough power.
if you twist and turn or drilling
you may go forward, upward etc
it is called zuan chuan/zhang (one of the 5 main palm/fist)
etc etc