The restriction for using a mutual effect is that it must do something. A cost (such as the Discard from Delcatty's energy draw) is not an effect but a cost.
For Delcatty ex, the effect is the discarding. If neither player has a hand, you can't use constrain simply so that it "counts as using a power".
This sounds a little obscure, but there are situations such as versus Alakazam MT (though not for the Delcatty ex example) or versus Shiftry ex CG (where damage on Delcatty ex may contribute towards an effect such as Rayquaza ex's poké-body or Scramble energy).
It's important to keep these things in mind and understand why you can or can't use a power such as Constrain.