elaborate on comment as requested.

src/cell.c

@@ -1722,7 +1722,8 @@ int cell_unskip_tasks(struct cell *c, struct scheduler *s) {
         if (l == NULL) error("Missing link to send_xv task.");
         scheduler_activate(s, l->t);
 
-        /* Drift both cells, the foreign one at the level which it is sent. */
+        /* Drift both cells, the foreign one at the level which it is sent, i.e.
+           drift the cell specified in the send task (l->t) itself. */
         cell_activate_drift_part(l->t->ci, s);
         if (t->type == task_type_pair) cell_activate_drift_part(cj, s);
 
@@ -1769,7 +1770,8 @@ int cell_unskip_tasks(struct cell *c, struct scheduler *s) {
         if (l == NULL) error("Missing link to send_xv task.");
         scheduler_activate(s, l->t);
 
-        /* Drift both cells, the foreign one at the level which it is sent. */
+        /* Drift both cells, the foreign one at the level which it is sent, i.e.
+           drift the cell specified in the send task (l->t) itself. */
         cell_activate_drift_part(l->t->ci, s);
         if (t->type == task_type_pair) cell_activate_drift_part(ci, s);