Add comment to explain how we ensure jumps on the second frame are ha…

…ndled correctly by NoJump

package/MDAnalysis/transformations/nojump.py

@@ -124,6 +124,12 @@ def _transform(self, ts):
             msg = f"Periodic box dimensions are not invertible at step {ts.frame}"
             raise NoDataError(msg)
         if ts.frame == 0:
+            # We don't need to apply the transformation here. However, we need to ensure we
+            # have the 0th frame coordinates in reduced form. We also need to set an
+            # an appropriate value for self.older frame. This is so that on the following frame
+            # we don't return early when we check `self.older_frame != "A"`. If we return early,
+            # then the transformation is not applied, and any jumps across boundaries that occur
+            # at that frame will not be accounted for.
             self.prev = ts.positions @ Linverse
             self.old_frame = 0
             self.older_frame = -1