That how it works, though. You could knock-back the ape whenever you want, but that's an awful idea. The knock-back pad works regardless of what the ape is doing (provided that you did not used it just before. As Texan was saying, there is a cooldown on the use of the pad, preventing you to use it too often)
Therefore, even if (let's say) you can use the knock-back pad every 30 sec, (even when the Ape is not casting anything), that's a very bad idea. Save the pad for when the ape is actually casting Unleash Fury (Not terrorizing roar)
Try it by yourself, but I'm nearly 100% sure of it. Sometime, if you're not careful to avoid terrorizing roar, the fear CC will drive you on the knock-back pad, KB'ing the boss before his Frenzy Cast, putting you in trouble because you're unable to use the pad again right after (because of the cooldown).
However, the wiki could be misleading when it say "Run out of range when the boss starts his fear cast(Terrorizing Roar. Run down to the knockback pad, because the boss will start a to cast Unleash Fury soon after the fear, the only way to avoid the boss casting the enrage is to use the knockback pad.". I guess it could give the impression that you have to run and USE the pad when he start terrorizing roar, but that's wrong. You have to wait for the Unleash fury casting.