It's my belief there are a whole range of factors that can affect a player's performance in one game or even one at-bat. Some of those factors may be physiological or psychological - the hitter just feels good, he's confident, alert, he's 'seeing the ball well', as the expression goes. Some of the factors may be mental or strategic - the hitter has studied this pitcher and has a good idea what he throws in certain situations. Some of the factors are luck and randomness - the hitter guesses right on the pitch, or he hits the ball through a hole in the infield, when he was basically just trying to make good contact.
When you've got so much stuff going on it's virtually impossible to say exactly why the hitter succeeded that time. As we all know even the best hitters only hit .300. And even the best clutch hitters can't keep coming up with big hits on a consistent basis because the odds are stacked against them.
With all that said I do consider Ortiz and Schilling to have been clutch performers in the postseason.