You (with the expired CFI and current ATP) can give instruction to someone in air carrier situations. So...perhaps you're showing your FO how the FMS really works...or how the Ops Specs apply to this situation or that.
-mini
*edit*
Here it is
61.167
(b) An airline transport pilot may instruct—
(1) Other pilots in air transportation service in aircraft of the category, class, and type, as applicable, for which the airline transport pilot is rated and endorse the logbook or other training record of the person to whom training has been given;
(2) In flight simulators, and flight training devices representing the aircraft referenced in paragraph (b)(1) of this section, when instructing under the provisions of this section and endorse the logbook or other training record of the person to whom training has been given;
(3) Only as provided in this section, unless the airline transport pilot also holds a flight instructor certificate, in which case the holder may exercise the instructor privileges of subpart H of part 61 for which he or she is rated; and
(4) In an aircraft, only if the aircraft has functioning dual controls, when instructing under the provisions of this section.
(c) Excluding briefings and debriefings, an airline transport pilot may not instruct in aircraft, flight simulators, and flight training devices under this section—
(1) For more than 8 hours in any 24-consecutive-hour period; or
(2) For more than 36 hours in any 7-consecutive-day period.
(d) An airline transport pilot may not instruct in Category II or Category III operations unless he or she has been trained and successfully tested under Category II or Category III operations, as applicable.