The end block executes after process has acted on all objects in the input pipeline.

The end block cannot use the $_ automatic variable. Parameters that accept pipeline input will be filled with the last value from the process block:

function Show-Pipeline {
    begin {
        $position = $myinvocation.PipelinePosition
        Write-Host "Pipeline position ${position}: Start"
    }

    process {
        Write-Host "Pipeline position ${position}: $_"
        $_
    }

    end {
        Write-Host "Pipeline position ${position}: End"
    }
}

Running this command in a pipeline shows the end executing after all items in the input pipeline have been processed:

PS> $result = 1..2 | Show-Pipeline
Pipeline position 1: Start
Pipeline position 1: 1
Pipeline position 1: 2
Pipeline position 1: End

Commands that make extensive use of the end block include Measure-Object, ConvertTo-Html, and ConvertTo-Json. Such commands cannot return output until the end because the output is only valid when complete. The same is true of any other command that must gather input during a process, and output something on completion.

A simple command to count the number of elements in an input pipeline is shown here. The process block is unable to determine this; it must run again and again until the input pipeline is exhausted:

function Measure-Item {
    begin {
        $count = 0
    }

    process {
        $count++
    }

    end {
        $count
    }
}