This summary is not the rule. The rule text controls.
Here's a plain English summary:
You don't have to wait your turn to gather evidence in a lawsuit. Both sides can send questions, request documents, or take depositions in whatever order works best for them. There's no required first step or set sequence you must follow.
If one side is in the middle of gathering evidence, that doesn't stop the other side from doing their own evidence gathering at the same time. Both parties can work on discovery at once instead of taking turns.
A judge can change this if there's a good reason. If following the normal free-for-all approach would be unfair or inconvenient for the people or witnesses involved, a party can ask the court to set a specific order or schedule instead. But unless a judge steps in and says otherwise, everyone is free to pursue evidence in any order and at the same time as everyone else.