The pressure is certainly not off if you're behind in the game. About as far from it as possible, in fact.
In terms of being a leader and dictating terms, the pressure is less if you're behind in the game. A bloke who only fires up in the second innings when his team is in strife has the advantage of bowling his guts out for a short period of time and if it works, great but if not, he can say "Well I bowled my guts out, we lost, not my fault." i.e. the pressure is really only on them to bowl well in the second innings and for a short burst at that. It's much easier to throw everything you've got at the opposition when you think you've got nothing to lose.
That's the difference, cognitively-speaking, between striving to be at your best all the time compared to when your behind in the match; the pressure of the imminent loss is there but you can erase the responsibility in your mind for your team's position in the match with one or two spells where you finally put in, thereby releasing the dissonance. Someone who strives to do well all the time feels the pressure to do so all the time and if their team loses, regardless of whether they bowled well in patches, they'll feel it afterwards and then when they front-up for the next Test. They only release that tension when their team wins.
The problems that people have with these sorts of blokes is multi-faceted;
a) It's low-percentage as a tactic. Waiting for motivation to materialise in the form of your opposition dominating you is dangerous and won't work most of the time as evidenced by England's relatively poor win-loss record and Caddick's not-as-good-as-it-should-have-been record.
b) For the same reason as above, letting your opponent dictate the course of the game will lose you more games than win.
c) It smacks of only getting into the game when you bloody well feel like it and is not usually the tactic of genuine team players. Guys like McGrath and Warne didn't need the threat of defeat to get them motivated to turn up. They just did their job and let their opponents chase them and not the other way around. Anyone playing at the highest level should not need any other motivation to do well or they shouldn't be playing Test cricket. Nothing saps the confidence of a team more than people who turn up to play on their terms only.
Someone who takes control of the game right from the start has the additional pressure of not only bowling well when the game is still fairly even and opponents are jostling for position but
also in the second innings unless their team absolutely demolishes the opposition. That's why, overall, the pressure is greatest on those who choose to come into the game in all conditions/points in a game.
As for Caddick specifically, I always thought he was a decent Test bowler who didn't play against Australia when at his peak. As Goughy said, when fired-up, he was pretty lethal and he was certainly hyped to the eyeballs before playing his first Test in 1993. I do reckon he used to phone-in his first innings performances sometimes but not as often as perception would indicate. Seemed to be harshly treated by selectors, though.