It's a myth....I didn't and still got pregnant. I found it much more efficient to get up and pee to prevent UTIs. The good swimmers are well on their way straight away. Legs up encourages the duffers in the right direction but you don't want them getting you pregnant anyway!
It's designed for periods, an expandable and reusable "cup" you insert after DTD so it holds sperm close to the cervix. Actually I think I ended up ordering a different brand called the Femmecup. You can get both on Amazon.
I figured it couldn't hurt (metaphorically - I'll let you know about physically!). I've read it does work well for some women who are having trouble TTC. As it holds the sperm close to the cervix for a longer amount of time, it gives the stronger swimmers increased odds.