  We consider a knot homotopy as a cylinder in 4-space. An ordinary triple
point $p$ of the cylinder is called {\em coherent} if all three branches
intersect at $p$ pairwise with the same index. A {\em triple unknotting} of a
classical knot $K$ is a homotopy which connects $K$ with the trivial knot and
which has as singularities only coherent triple points. We give a new formula
for the first Vassiliev invariant $v_2(K)$ by using triple unknottings. As a
corollary we obtain a very simple proof of the fact that passing a coherent
triple point always changes the knot type. As another corollary we show that
there are triple unknottings which are not homotopic as triple unknottings even
if we allow more complicated singularities to appear in the homotopy of the
homotopy.
