Timing belt marking pens

[Nukeman7]

I thought the tensioner side WAS the slack side (the tensioner takes up the slack when you pull the pin)

You are correct...he meant the red side is the tensioned side (i.e, taut before releasing the hydraulic tensioner).

 

[The Critic]

I thought the tensioner side WAS the slack side (the tensioner takes up the slack when you pull the pin)

Tension* side, I just made the edit. Thanks.