You've got to be pretty weak first, before you try it.

If you try it when you are strong, you make a failure of it, and then that queers you for ever after.

I made the mistake of telling Jake the trick when he was strong.

Of course, he could not pull it off, and in the times since when he did need it, it was too late, for his first failure had queered it.

He won't even believe it now.