You can choose to extend the duration of an effect created by a wizard at-will spell (such as cloud of daggers or ray of frost) that would otherwise end at the end of your current turn. The effect instead ends at the end of your next turn
this only works with power that have a lasting effect, a nice detail is that you chose to use this power not on the round that you cast the spell but on the round it would end.
round 1: cast spell (cloud of daggers)
round 2: do your normal actions in this round, normaly coud of daggers would end on the end of this turn, but you can chose to make it last longer ( you can look if there aee still creatures hampert by it before making the choice)
round 3: take your normal actions on the end of this round the spell (coud of daggers) ends