The process for yielding is different on the first iteration of a CustomYieldInstruction...

1. it checks the yield
2. places the coroutine in a queue no matter what and waits
3. once it starts processing the coroutine, if the previous yield returned false then it continues the coroutine.
4. if, however, the previous yield returned true then it does...
5. another yield check to verify
6. if this time the new yield is false, then it continues the coroutine.
7. if, however, the new yield is true, then waits and jumps back to 5.