I've got a challenge...

Alright. So you guys all fairly familiar with the lasso tool in Photoshop and Paint and stuff? Basically it lets you create free-hand selections. I remember playing some old games that used this kind of selection sysytem to destroy enemies and such, so I'm now trying to create a game which utilizes a similar system. The problem is, I've hit a brick wall. I can't figure out the theory behind creating one. Once I know what to do I'm sure I can code it, but I can't think of anything that would ALWAYS work.