Like in the following sentence:
I know that ~てくれ is a command, but I don't understand what that ~や means
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～てやる means 'do something (for someone of equal or lower social standing than you)'; it doesn't emphasize the command. ～てくれ is a command asking them to do something for you. With both used together like this, the sentence roughly means, 'Fujioka, do me a favour and tell them all about it.' It does sound pretty masculine because ～てやる can sound that way and ～くれ is an imperative.
やっ comes from やる, which is conjugated to やって in the continuous verb pattern.
～てやって by itself is a strong/rough way of saying 'do ______.'
～てくれ is also, by itself, a strong/rough way of saying 'do _____.'
When you double them up and say ～てやってくれ it strengthens the command (or strong request) even more, so that there is no room for misunderstanding.
This manner of speech can be seen as overly aggressive, so take care with its use.
@Aeon Akechi has a good point as to how it would be used vis a vis instructions regarding third parties. 'Do ____ for me, くれ.' 'Teach them/tell them for me, please.'.
It is not always as severe as I described above, so let context to be your guide.