I meant jumpy stuff really, and stuff like practising trackstands, which I still can't do, and wheelies, which last about half a turn of the pedals.... and yes, if there's a chance of falling off or over I think flats are best. On technical stuff though I much prefer being "in touch with" the bike. I used to ride with DX style pedals, with a big surround so I could dab and get back onto the pedals, and occasionally unclipped for scary rocky descents, but I eventually realised that I didn't feel anywhere near secure enough if my feet could be bounced off the pedals by rockstrikes etc. My other personal advantage is ruts and so on, especially on tech climbs, where occasionally you won't get the pedals to do a full revolution. Clipped in you can toe tap AND pull up as a get out of jail trick.
What we mustn't do is give people, especially novices, the idea that riding clipless means you are going to fall off more, otherwise it would be counterintuitive to use SPDs on anything other than an absolutely smooth surface. We never seem to mention that the main disadvantage of flat pedals is the increased likelihood of shin rippage from pins, and general shin damage through losing the pedals in rough stuff.
SO, if you are starting out on a MTB, my personal recommendation would be to get some decent SPDs (please don't get M520s, there are better pedals available for not much more money....) and ride progressively rougher and gnarlier trails in them until you gain complete confidence. It'll seem hard work at first, but persevere and hone your technique, until unclipping and -more difficult on rough stuff- clipping back in is a piece of cake.
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