EIGRP and poison reverse

Today, Stretch posted an interested EIGRP update scenario.  I found myself confused, though, at Step 4 where it says "R3 has a learned of a new path to 0.0.0.0/0 from the reply sent by R2. First, it sends a poison reverse update to R2 for this route."  Why would a router send a poison-reverse update for a route to a neighbor whom originated the route?  Cisco sheds more light

Thanks Stretch.

dynamic backup routes using Cisco's Enhanced Object Tracking

     In this example, routerA has two ethernet interfaces which both connect to service providers. Using traditional static routes with different metrics would only remove the primary route if the interface itself were to go down (i.e. "line is down,protocol is down"). If the S.P. on the primary link had a routing issue or some other problem (janitor tripped over a power cable 200 miles away), routerA needs to update its routing table even though its interface is still up. Enter starge left Cisco's object tracking.

     RouterA has a primary ethernet interface at Fa1/0/1 and a backup at Fa1/0/2. We create a policy map to force pings to a certain address (something close to your critical destinations, or the the Internet at large if there's no specific traffic you want to track) to use the primary interface (no, I don't understand why it's routed to Null 0) and when that fails, it removes the default route. The secondary floating route then takes over.

!!begin sample config
hostname routerA
ip subnet-zero
ip routing
!
ip sla 1
icmp-echo 4.2.2.2
request-data-size 1400
timeout 2000
threshold 2000
frequency 3
ip sla schedule 1 life forever start-time now
!
interface FastEthernet1/0/1
description Primary Link - .1 is the default gateway for this subnet
ip address 172.16.0.2 255.255.255.0
!
interface FastEthernet1/0/2
description Backup Link - 10.0.0.1 is the gateway for this subnet
ip address 10.0.0.2 255.255.255.0
!
ip local policy route-map MY-LOCAL-POLICY
!
access-list 101 permit icmp any host 4.2.2.2
route-map MY-LOCAL-POLICY permit 10
match ip address 101
set interface Null0
set ip next-hop 172.16.0.1
!
ip route 0.0.0.0 0.0.0.0 172.16.0.1 track 100
ip route 0.0.0.0 0.0.0.0 10.0.0.1 254
!
track 100 rtr 1 reachability
!
!! end sample config

Details on Cisco's Enhanced Object Tracking feature can be found at http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html#wp1071672 .