Since flaxseed oil is a fat (a good fat nonetheless because it's an EFA), it digests more slowly and makes you feel satiated longer. I've never heard that it actually slows digestion.
For some reason, the body needs EFA's (essential fatty acids) to burn body fat. I'm not sure why, but it works for me and a lot of other people on this board. EFA's do a lot of other good things for the body too, like lower cholesterol, make your skin and hair nice, etc.
Just remember to count the calories you get from flaxseed oil as part of your total fat calories.