async void Form1_Load(object sender, EventArgs e)
{
    // on the UI thread
    Debug.WriteLine(new { where = "before",
        Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread });

    var tcs = new TaskCompletionSource<bool>();

    this.BeginInvoke(new MethodInvoker(() => tcs.SetResult(true)));

    await tcs.Task.ContinueWith(t => {
        // still on the UI thread
        Debug.WriteLine(new { where = "ContinueWith",
            Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread });
    }, TaskContinuationOptions.ExecuteSynchronously).ConfigureAwait(false);

    // on a pool thread
    Debug.WriteLine(new { where = "after",
        Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread });
}

await

await Task.Delay(100).ContinueWith(t =>
{
    // on a pool thread
    Debug.WriteLine(new { where = "ContinueWith",
        Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread });
}, TaskContinuationOptions.ExecuteSynchronously).ConfigureAwait(false);

ConfiguredTaskAwaitable

class DumbSyncContext: SynchronizationContext
{
}

// ...

Debug.WriteLine(new { where = "before",
    Thread.CurrentThread.ManagedThreadId,
    Thread.CurrentThread.IsThreadPoolThread });

var tcs = new TaskCompletionSource<bool>();

var thread = new Thread(() =>
{
    Debug.WriteLine(new { where = "new Thread",
        Thread.CurrentThread.ManagedThreadId,
        Thread.CurrentThread.IsThreadPoolThread});
    SynchronizationContext.SetSynchronizationContext(new DumbSyncContext());
    tcs.SetResult(true);
    Thread.Sleep(1000);
});
thread.Start();

await tcs.Task.ContinueWith(t => {
    Debug.WriteLine(new { where = "ContinueWith",
        Thread.CurrentThread.ManagedThreadId,
        Thread.CurrentThread.IsThreadPoolThread});
}, TaskContinuationOptions.ExecuteSynchronously).ConfigureAwait(false);

Debug.WriteLine(new { where = "after",
    Thread.CurrentThread.ManagedThreadId,
    Thread.CurrentThread.IsThreadPoolThread });

SynchronizationContext

await FooAsync().ConfigureAwait(false);

static Task FooAsync(bool runSync)
{
   if (!runSync)
       await Task.Delay(100);
}

await FooAsync(true).ConfigureAwait(false);

await task.ConfigureAwait(false);