A very good video explaining the root causes of tennis elbow.
However, doczac is promoting this product Handmaster Plus which is an exercise tool to strengthen the finger extensor muscles.
In short, tennis elbow is chiefly cause by repetitive use of the fingers to grip the racquet. Particularly gripping it too tight.
The grip size is particularly important, the thinner grip usually cause one to grip more tightly than a thicker grip.
This website explained very clearly the rationale behind questions about racquet science. (biasness towards tennis, though)
Here Anna Rice explains some stretching that can help to alleviate this problem.