Understanding backpressure

The only problem with Observable is when an Observer cannot cope with the pace of an Observable. An Observable, by default, chains work by pushing items synchronously to the Observer, one at a time. However, if the observer has to perform some time-consuming computations, this may take longer than the interval of each item emission of Observable. Confused? Let's consider this example:

    fun main(args: Array<String>) { 
      val observable = Observable.just(1,2,3,4,5,6,7,8,9)//(1) 
      val subject = BehaviorSubject.create<Int>() 
      subject.observeOn(Schedulers.computation())//(2) 
            .subscribe({//(3) 
                println("Subs 1 Received $it") 
                runBlocking { delay(200) }//(4) 
            }) 
 
            subject.observeOn(Schedulers.computation())//(5) 
            .subscribe({//(6) ...