According to the full activity timeline, the two communication sessions before and after the closest approach are uplinks, not downlinks. Presumably these are used to provide refined targeting based on the preview observations.
There are also DSN tracking sessions after the uplinks, which don't correspond to any returned results. I guess that either
- the REX transmitter can transmit a carrier wave during an uplink, but can't encode data on it, or
- the DSN antennas are "illuminating" the spacecraft with sideband signals during the uplink, and then looking for a passive reflection after the round trip.
The first session:
July 13 21:00:00 EDT
New Horizons soon to be communicating with Madrid 70 m (DSS-63)
July 13 23:30:00 EDT
New Horizons' track ending with Madrid 70 m (DSS-63)
July 14 01:57:59 EDT
New Horizons is taking radio uplink data with REX. New Horizons is 31.8 AU from Earth.
July 14 02:05:00 EDT
New Horizons soon to be communicating with Goldstone 70 m (DSS-14)
July 14 06:40:00 EDT
New Horizons' track ending with Goldstone 70 m (DSS-14)
The second session, overlapping the first:
July 14 03:15:00 EDT
New Horizons soon to be communicating with Canberra 70 m (DSS-43)
July 14 06:40:00 EDT
New Horizons' track ending with Canberra 70 m (DSS-43)
July 14 09:28:02 EDT
New Horizons is taking radio uplink data with REX. New Horizons is 31.9 AU from Earth.
July 14 10:55:00 EDT
New Horizons soon to be communicating with Canberra 70 m (DSS-43)
July 14 12:00:00 EDT
New Horizons' track ending with Canberra 70 m (DSS-43)
I don't see any REX activity corresponding to the last Canberra tracking, so I guess that indicates they're looking either for reflection of signals from the earlier part of the preceding track, 9 hours earlier, or simply looking for it without any illumination.
Also, the simulator's depiction of a 2.5 hour radio session immediately following closest approach disagrees with that timeline. Perhaps such a session was scheduled to conservatively return results ASAP, before passing through the entire system, but then they decided to perform more observations instead and delay the First Look returns.