Using seed to send bitcoin from a wallet's specific private key
You can use any BIP39 compatible software to enter a mnemonic (12-24 words) and turn it into a seed. Seed is just a number between 1 and 2^256. You can then use this seed to create a Master Private Key. This key then serves as the root of a chain used to create other private keys based on a derivation path.

The whole process is deterministic so same mnemonic and same path will always lead to the same key. Once you have a normal private key you can import it to any bitcoin client and access the funds as usual.