Settings take effect as soon as the “Updating information” message in the app stops.
There are some merchant types where a merchant will pre-authorize the card for an amount that may be larger than the actual transaction amount. In this instance the pre-authorization amount must meet the threshold spend limit.
Controls are only invoked during authorization of a transaction. In some cases, such as Gas Stations, a card may be tested for validity by doing a $1 pre-authorization, and the actual transaction amount is charged to the debit card after the transaction. Some gas stations will preauthorize for a maximum amount, e.g. $126, and some gas stations will check validating, e.g. $1. In the latter case, the actual transaction amount may exceed the limit, while in the former case, an authorization may be denied.