I'll simply request that you watch the video. It tells it as good–if not better–than I can.
I will sum it up this way:
If you want to ask someone for a big favor or a big purchase, then ask them for a smaller one first.
For example, if you want someone to buy a high-ticket coaching program, you first ask them to buy a small report. Once they say “Yes,” they will be more inclined to say “Yes” again, even if the next request is a bigger “ask.”
This also falls into that horrible concept of a “trip wire.”
The interesting thing is when you turn this concept on its head, by asking for a really huge thing first, then asking for what you really want, you often also get what you want.
This is called the “Door in the face” technique!