Why it works
Receiving an unexpected extra gift triggers a natural feeling of indebtedness in the recipient. This small, unrequested favor encourages them to be more receptive to your larger request.
Example
I've included a free cheat sheet on conversion optimization to get you started.
An illustration of the mechanism, not a template. Copied verbatim it stops being persuasion and starts being the pattern everyone else is already sending.
Is your sequence pulling this one?
Paste it in and find out which levers are working, which are missing, and which single zero is capping the whole message.