From SmashWiki, the Super Smash Bros. wiki
Desynching is an advanced Ice Climbers technique that involves separating the two climbers' states and controlling them individually. It is most commonly done during grabs to allow Nana to attack a captive opponent or regrab them, but the Ice Climbers can be desynched out of many attacks, allowing the player to perform an attack, move, jump, or wavedash with either Nana or Popo, whilst inputting a different command to the other Ice Climber.
Continuous desynch, as demonstrated in Two Climbers, Two Hands is a technique and strategy which involves keeping the Climbers acting individually for extended periods of time. In addition to making the Climbers much more powerful offensively, they also become harder to approach with continuous desynch. Due to the extreme difficulty of timing and maintaining a continuous string of attacks, the in-combat potential of this approach is clearly substantial, but largely untapped.
Due to the increased popularity of the Ice Climbers, desynching has quickly become a common tactic for a number of Ice Climber mains. Videos have been created solely for the purpose of demonstrating the possibilities of this technique.