if ((Get-PSSnapin "Microsoft.SharePoint.PowerShell" -ErrorAction SilentlyContinue) -eq $null)
{
Add-PSSnapin "Microsoft.SharePoint.PowerShell"
}

$d = [Microsoft.SharePoint.Administration.SPWebService]::ContentService.DeveloperDashboardSettings
$d.DisplayLevel = 'OnDemand'
$d.TraceEnabled = $true
$d.Update()