Loop and boolean

Function Get-Acronym() {
    [CmdletBinding()]
    Param (
        [string]$Phrase
    )
    $newWord = $true
    foreach ($char in $Phrase.ToCharArray()) {
        if ($newWord -and $char -match "[a-z]") {
            $acronym += $char
            $newWord = $false
        }
        if (-not $newWord -and $char -match "[_\- ]") {
            $newWord = $true
        }
    }
    $acronym.ToUpper()
}

This approach use a flag with a boolean value to mark when a new word is about to begin or not.

    $newWord = $true

We then loop over the characters from the input using foreach and .ToCharArray() method. If we are about to start a new word (true) and the current char is a letter, then we add that letter to the result, and switch the flag to false. If we are not in phase of starting a new word (false) and the current char match one of the delimiters, then we switch the flag to true, signify that we are ready to start a new word.

    foreach ($char in $Phrase.ToCharArray()) {
        if ($newWord -and $char -match "[a-z]") {
            $acronym += $char
            $newWord = $false
        }
        if (-not $newWord -and $char -match "[_\- ]") {
            $newWord = $true
        }
    }
    $acronym.ToUpper()

After the loop is finished, we call ToUpper() method on the result and got our acronym in all capitalized letters.

Booleans.

Foreach loop.