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If you want to add your own WhatIf and Confirm parameters, you don't need to implement them like normal parameters, shown as follows:

# Do not do this
function Test-WrongRiskMitigation
{
    param
    (
        [switch]
        $WhatIf
    )

    if ($WhatIf)
    {
        Write-Host "Simulating shredding of evidence"
    }
}

Instead, you can make use of the CmdletBinding attribute, which automatically enables all common parameters and is able to add WhatIf and Confirm properly:

function Test-RiskMitigation
{
 [CmdletBinding(SupportsShouldProcess)]
 param ( )

 if ($PSCmdlet.ShouldProcess('Target object, here: The evidence','Action, here: Shred'))
 {
 Write-Host -ForegroundColor Red -Object 'Shredding evidence...'
 }
}

Test-RiskMitigation -WhatIf
Test-RiskMitigation

This way, you can simply react to the cmdlet emitting a confirmation message as a user. There's no additional implementation necessary apart from using the cmdlet binding and the automatic PSCmdlet variable.