Stemming requires some balance, technique and flexibility, but once you get it, it feels awesome (and looks good too!).
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The concept is simple - jump!
The technique is harder, and takes confidence. The easiest way to learn is on an indoor bouldering wall with large handholds and a big, soft mat to land on.
The tricky part is choosing the right moment - just at the peak of your jump - to let go of the lower holds and grab the higher ones. A common mistake is to do this too early, just at the outside of your circular path, because then your arms will not pull you back to the wall and you will fly off at a tangent. Of course letting go too late is also bad. You just have to learn by experimenting.
This has a 'camming' effect and locks the fingers into place - you can relax the muscles now and the hand will stay put.