When you are referring to something tangible, the choice between "this" or "that" is usually about proximity:
"This" is usually used when something is right in front of you. If your friend was standing next to you and you introduced them to somebody else, you would say "this is my friend".
"That" is usually for something that is not immediately in front of you. If your friend was at the other side of the room and you pointed them out to somebody you would say "that is my friend".
For events in time, "this" is usually used in connection with the present:
- "This is a bad year for me".
When something has left, or an event is in the past, you would normally use "that":
- "That was a bad year for me".
In your examples, you should try and match the tense of the previous statement, so:
Dolphins have a smooth, streamlined body. This helps them accelerate up to 34.5 mph.
Because you said Dolphins have a smooth, streamlined body, you are speaking in the present tense, so it is natural to say "this".
If your previous statement had been about something historical, then it would be more appropriate to use "that", for example:
Tyrannosaurus rex had tiny hands. That must have made it difficult to pick things up.
Your second example is speaking about something that occurs in writing, and although it happens only "sometimes", the statement "diminutives supersede official names" is in simple present tense. For that reason, I would also favour "this":
Sometimes, even in formal writing, diminutives supersede official names. Examples of this include the name of the 39th president of the United States Jimmy Carter.