A doubt about concept no. 119 in Solidity201

[toddteller]

Hello everyone,

This is the concept:

119. Storage Layout & Types: It might be beneficial to use reduced-size types if you are dealing with storage values because the compiler will pack multiple elements into one storage slot, and thus, combine multiple reads or writes into a single operation.

a. If you are not reading or writing all the values in a slot at the same time, this can have the opposite effect, though: When one value is written to a multi-value storage slot, the storage slot has to be read first and then combined with the new value such that other data in the same slot is not destroyed.

"if you are not reading or writing all the values in a slot at the same time"

How can you write all the values in a slot at the same time? If not, is the opposite effect inevitable when writing?