frame blending help

Frame blend if you're slowing down the speed. Might also apply some motion blur, depending on the speed. Don't frame blend if you're speeding up.

Separate action in the motion tab. Play with it for best results.

could someone explain to me what 'frame blending' actually does when slowing a >clip down?

If you want a clip to play for 4 seconds that is only 2 seconds long, you will need extra frames, duplication adds the new ones.
Frame blending does more than a straight duplication, it uses the before and after frames to create intermediate frames and will give a smoother look, but sometimes a little unatural in appearance.

Try turning it off if you don't like the look.

Al