PowerShell: Toggle "Num Lock" on and off.

I experienced the same on/off flicker you did.

This works fine for me though, give it a try:

$wsh = New-Object -ComObject WScript.Shell
$wsh.SendKeys('{NUMLOCK}')

For what it is worth, from the perspective of the keyboard, the OS sends a set and then a reset of the toggle key (caps, scroll lock, numlock) when using [System.Windows.Forms.SendKeys]::SendWait("{CAPSLOCK}"), but only sends a single event using WScript.Shell.