holding it in actually does get you 'higher'
i've been doing some studying and researching at the Library just up the block from me... along with my own personal tests and my doctors opinion. the smoke carry's the THC. THC is the main component that gets you high. if you take in more smoke, you're taking in more THC, if you blow the smoke out instantly, all you're taking in is the THC that was immediately absorbed by your lung walls (most of the time just a thin coat, so to speak) but if you hold it in, it absorbs more of that THC. (Obviously?)
now for my own personal tests, i took a hit from a joint, held it for about 5 seconds. i didn't feel anything, i waited for a half an hour, nothing. After i waited 30 minutes, i took another hit from the same joint, held it in for about 15 seconds, and within 5 minutes i was already feeling a slight buzz.
and my medical marijuana doctors' opinion is that (as stated above) if you hold in that smoke, you're holding in that THC, and your lungs can actually hold more THC then you think.
i do and always will hold my hits in for as long as i can, it's always without fail gotten me more high.