I find I put my non-throw hand thumb into the trapeze loop and push the string back towards me.
On my throw hand, I “make the gun”, and then turn it palm up and move my throw hand slightly towards me relative to the non-throw hand(or move the non-throw hand forward). Make sure you have a fair amount of string on the looped side, around half, or you’ll get the rejection landing on both strings or just the front.
If you do that in a trick ladder contest, the trick won’t count. What I do is like it says in the yotricks.com video. I point my freehand index finger down and move it a little forward to hit the back string. I’ve heard that from more than one expert. Also, on my yoyo hand, I separate my index finger and thumb to make it “drop” onto the back string. There’s a trick called “Candy Rain” that uses the magic drop in the front. Check it out. http://www.youtube.com/watch?v=_t6mU9Qx6DE