Clarify derived signal tracking behavior

Author: Janani-bn

src/routes/(0)concepts/(2)derived-values/(0)derived-signals.mdx

@@ -13,21 +13,34 @@ tags:
   - state
 version: "1.0"
 description: >-
-  Create reactive derived values that automatically update when their
-  dependencies change using Solid's derived signals.
+  Derive values from signals with plain functions and use them in tracking
+  scopes to keep dependent computations up to date.
 ---
 
 Derived signals are functions that rely on one or more [signals](/concepts/signals) to produce a value.
 
-These functions are not executed immediately, but instead are only called when the values they rely on are changed.
-When the underlying signal is changed, the function will be called again to produce a new value.
+A derived signal is a plain function: it runs when you call it, not automatically when a signal changes.
+Each call reads the current signal values and calculates a new result.
 
 ```js
 const double = () => count() * 2;
 ```
 
-In the above example, the `double` function relies on the `count` signal to produce a value.
-When the `count` signal is changed, the `double` function will be called again to produce a new value.
+Calling `double()` reads `count()` and returns twice its current value.
+Changing `count` alone does not call `double`.
+If you call `double()` inside a tracking scope, such as an effect or a JSX expression, that scope tracks the `count()` read and runs again when `count` changes.
+
+```js
+import { createEffect, createSignal } from "solid-js";
+
+const [count, setCount] = createSignal(1);
+const double = () => count() * 2;
+
+console.log(double()); // 2: a one-time read
+createEffect(() => console.log(double())); // tracks count
+
+setCount(2); // the effect sees the new value; the one-time read does not run again
+```
 
 Similarly you can create a derived signal that relies on a store value because stores use signals under the hood.
 To learn more about how stores work, [you can visit the stores section](/concepts/stores).
@@ -36,9 +49,11 @@ To learn more about how stores work, [you can visit the stores section](/concept
 const fullName = () => store.firstName + " " + store.lastName;
 ```
 
-These dependent functions gain reactivity from the signal they access, ensuring that changes in the underlying data propagate throughout your application.
-It is important to note that these functions do not store a value themselves; instead, they can update any effects or components that depend on them.
-If included within a component's body, these derived signals will trigger an update when necessary.
+Derived signals do not store or cache their results.
+Calling one multiple times repeats the calculation each time.
+Reading one at the top level of a component is a one-time read, since components do not re-run when signals change.
+Use it inside JSX or another tracking scope when the result needs to stay up to date.
 
-While you can create derived values in this manner, Solid created the [`createMemo`](/reference/basic-reactivity/create-memo) primitive.
+For a cached derived value, use [`createMemo`](/reference/basic-reactivity/create-memo).
+A memo tracks its own dependencies and reuses its result until those dependencies change.
 To dive deeper into how memos work, [check out the memos section](/concepts/derived-values/memos).