在您的示例中，由于：

sentenceSource: akka.stream.scaladsl.Source[String,akka.NotUsed] = ???
wordCounter: akka.stream.scaladsl.Flow[String,Int,akka.NotUsed] = ???

两者都将NotUsed作为其物化（指示它们没有有用物化），

sentenceSource.viaMat(wordCounter)(Keep.right)
sentenceSource.viaMat(wordCounter)(Keep.left)

但是，由于Sink.head[T]物化为Future[T]，因此更改组合器显然会产生影响

val intSource = sentenceSource.viaMat(wordCounter)(Keep.right)

val notUsed = intSource.toMat(Sink.head)(Keep.left)
// akka.stream.scaladsl.RunnableGraph[akka.NotUsed]

val intFut = intSource.toMat(Sink.head)(Keep.right)
// akka.stream.scaladsl.RunnableGraph[scala.concurrent.Future[Int]]

notUsed.run    // akka.NotUsed

intFut.run     // Future(Success(12))

Source中的大多数源实体化为NotUsed，几乎所有常见的Flow操作符也是如此，所以toMat(someSink)(Keep.right)（或等价的.runWith(someSink)）比使用Keep.left或Keep.both要普遍得多。源/流实体化最常见的用例是提供某种控制平面，例如：

import akka.Done
import akka.stream.{ CompletionStrategy, OverflowStrategy }

import system.dispatcher

val completionMatcher: PartialFunction[Any, CompletionStrategy] = { case Done => CompletionStrategy.draining }
val failureMatcher: PartialFunction[Any, Throwable] = { case 666 => new Exception("""\m/""") }

val sentenceSource = Source.actorRef[String](completionMatcher = completionMatcher, failureMatcher = failureMatcher, bufferSize = 100, overflowStrategy = OverflowStrategy.dropNew)

// same wordCounter as before
val stream = sentenceSource.viaMat(wordCounter)(Keep.left).toMat(Sink.head)(Keep.both)    // akka.stream.scaladsl.RunnableGraph[(akka.actor.ActorRef, scala.concurrent.Future[Int])]

val (sourceRef, intFut) = stream.run()

sourceRef ! "Materialized values are confusing me"
sourceRef ! "I love streams"
sourceRef ! "Left foo right bar"
sourceRef ! Done

intFut.foreach { result =>
  println(result)
  system.terminate()
}

在本例中，我们使用Keep.left来传递sentenceSource的物化值，然后使用Keep.both来同时获得该物化值和Sink.head的物化值。