Triangulation #3: Being at the right moment at the right spot
Triangulation is a tactic used in chess to put one's opponent in zugzwang. That is, it gets to the same position with the other player to move, ...
(Triangulation (chess), wikipedia)
The above definition is not a perfect one. Sometimes triangulation is better described as
I) 'Being at the right moment at the right spot'.
or even better
II) 'Not being at the wrong moment at the wrong spot'
Now Black's king shouldn't be on g6 (or e6) if the White's king can reach f4 (that is: on e3 and f3). In that case White moves his king to f4 and Black must leave g6 (or e6). But by leaving g6 (or e6) Black gives up the defence of f5.
On the other side, though more complicated: White's king shouldn't be on f4 if the Black one can reach e6.