Internal Transactions
definition
Core idea
An internal transaction is a transfer that originates from within a smart contract rather than directly from a wallet. A smart contract is an automated workflow for money, so when you interact with one it can move ETH on your behalf. These appear on a separate explorer tab and are often where a funding source is hidden.
When to use
When a wallet appears to have no normal incoming transaction explaining its balance: check the internal transactions tab, since the funding may have come out of a contract such as a mixer.
Example
An address funded by Tornado Cash shows the incoming value under internal transactions, because the withdrawal was initiated from the Tornado Cash contract.
Related
Address View vs Contract View on a Block Explorer, Tornado Cash to RenBTC Cross-Chain Laundering Pattern