Concept demo: array shift

arrayshift.ps1

<#
$step < 0 - left shift
$step = 0 - reverse
$step > 0 - right shift
#>
function Shift-Array {
	[cmdletBinding()]
	param (
		[Parameter(ValueFromPipeline)]
		$Inputobject,
		[int]$Step
	)

	begin {$collection = [System.Collections.Generic.List[object]]::new()}
	process {
		if ($InputObject -ne $null) {$collection.addrange(@($InputObject))}
	}
	end {
		if ($collection.count -lt 2) {return $collection} # it takes two or more to shift
		$alength = $collection.count
		if (-not $step) {$step = $alength-1} # default

		# prevent extra loops; adjust shift step
		if ($step -gt 0 -and $step -gt $alength) {$step = $step % $alength}
		elseif ($step -lt 0 -and -$step -gt $alength) {$step = $step % $alength}
		$ustep = [math]::abs($step)
		if ($ustep -eq $alength) { # 360deg : start = stop
			return $collection
		} elseif ($ustep -eq ($alength-1)) { # default
			[array]::Reverse($collection)
			return $collection
		}

		if ($step -lt 0) { # left shift
			for ($s=-$step; $s -gt 0; $s--) {
				$first = $collection[0]
				for ($i=1; $i -lt $alength; $i++) {
					$collection[$i-1] = $collection[$i]
				}
				$collection[-1] = $first
			}
		} 
		else { # right shift
			for ($s=$step; $s -gt 0; $s--) {
				$last = $collection[-1]
				for ($i=$alength-1; $i -gt 0; $i--) {
					$collection[$i] = $collection[$i-1]
				}
				$collection[0] = $last
			}
		}
		$collection
	} # end
} # END Shift-Array

I would suggest change testing on $null.
Or
place $null in the begin
if ($InputObject -ne $null) {$collection.addrange(@($InputObject))}
into
if ($null -ne $InputObject) {$collection.addrange(@($InputObject))}
Or
if ([string]::NotNullOrEmpty($InputObject)) {$collection.addrange(@($InputObject))}