The coxswain's role during indoor training season can be an ambiguous one, especially going through it the first season as the ergs rev up like jet turbines and any small objects nearby get sucked into the intake. "It's rough for coxswains," a rower once told me, "because they don't really have a role in the practices. Personally, I don't like being coxed on the erg, and usually tell people to stay away, so I imagine every coxswain is sort of questioning their role right now."
It's not that there isn't one; it's just harder to find. Indoor training is as much a learning time for coxswains as for rowers. Listen to the coach, particularly in the tanks; it's a great opportunity to see rowing from angles outside the boat a coxswain seldom sees, and to observe how the technique translates from word to motion. Assist the coach in running the workouts, recording stats, times, keeping the workouts efficient, and helping to keep people focused on their goals (this is good practice for next season in the boat). By tacit agreement, keeping the erg room CD player well-fed, promptly changed, and semper vitalis is often an official/unofficial duty of the coxswain. Badger your rowers about stretching properly; then badger them some more (tell 'em I sent ya) -- no such thing as too much flexibility.
Indoor training also lets you know your rowers one-on-one as each overcomes personal thresholds of pain and fatigue, and gives you a fine appreciation of their commitment in doing so. Watching your rowers erg a redline piece will teach you as much about determination and breaking barriers as a month on the water. Take your cue from watching your captain. Be active and encouraging; practice motivating without yelling; never lounge around. Project that "cone of intensity" on land just as you would on the water; this too is good training for when you get back in the boat. Without having to worry about steering, you can concentrate a lot more on the actual sound of your calls.
Many rowers do not want to be coxed on the erg (and may threaten to punch your head in if you try). Erging can be a very personal battle of self against machine with no room for a third person; you don't want to get between that particular hammer and anvil.
However, a few rowers find being coxed helpful under certain circumstances. Let your rowers know you are glad to cox erg pieces for them as a service on request, but do not force it. Some rowers prefer to be coxed by other rowers. If one of your rowers does request it, talk through the piece ahead of time so that you know exactly what is wanted. Be wary of trying to cox technique on the erg unless you are very sure you know exactly what you are doing; mistaken advice can lead to bad habits. That said, reminders of slide control are usually helpful because the mechanics of the erg tempt slide rush even in people who otherwise would never do it.
Bench pulls are the exception -- everyone I have ever known gets a rush from having their teammates cheer them on.
During erg pieces -- whether you are saying anything or not -- always stand in front where the rowers can see you. Erging with someone hovering behind you is incredibly distracting.
Do the land workouts with your rowers, especially runs (and whatever else you do to keep your weight down). Your erg scores might not impress anybody -- not even other coxswains -- but your effort shows your commitment to the boat, and rowers will respond to you much better on the water for having "been there" with them.