Learn the time trial position: how low you should really sit, why upright can be faster below 30 km/h and how to build the aero position in 5 steps.
The honest answer: the lower you sit, the less drag you have, but the less power you produce. There is no "as low as possible", only a point in between that depends on your speed. That is the core of the aero position. In a study with 19 well-trained men on an ergometer, five torso angles were compared (their own preferred one plus 0, 8, 16 and 24 degrees). With a lower torso, peak power and the respiratory and metabolic values dropped [1]. The authors speak of a trade-off between aerodynamics and physical performance [1].
How low is sensible depends on speed: a model from the same data found that drag outweighs power losses above 46 kilometres per hour, but a fully horizontal torso is never optimal; below 30 kilometres per hour a more upright position is even advantageous [2]. That means: in a recreational time trial at 35 to 40 kilometres per hour your optimum is probably somewhere between upright and all the way low (an inference from [2], not from your measurement).
You do not win aero through flatness alone but through a position you can hold and still pedal in.
The price also shows in the field: nine competitive men rode three 40-kilometre time trials: two in the lab (once in an aero and once in an upright position) and one on the road. In the aero position their power was significantly lower than upright and than on the road [3]. Realistically you need several weeks until you can hold the position for 20 to 30 minutes (experience value).
Set saddle and aero bars so that you can hold the position without neck or back pain. Saddle usually a bit further forward than on a road bike (practice standard).
Put your forearms on the pads, hands loose on the grip. Elbows are under the shoulders. Practise this first at standstill or on a quiet, traffic-free road.
Milestone (practice): 5 minutes of aero position on a free road without pain.
Start with a medium position and lower the torso only as far as you can still pedal hard. Studies show that power drops with a lower torso [1]. Measure whether the watts stay the same, not just whether it looks low.
Pay attention to a round pedal stroke. The hip must not be pinched, otherwise power is missing. Breathe deeply: with a lower torso, oxygen uptake and ventilation also dropped in the test [1].
Milestone (practice): 20 minutes continuously in the position at even power.
For corners, braking and obstacles you return to the base bar. If you want to brake or steer in the aero position you need time that you do not have. Plan the switches before corners and junctions.
On aero bars you do not reach the brakes immediately and steer only to a limited degree. Ride the aero position only on free, open roads without traffic, never in groups or in a bunch. Whether the aero position causes more injuries, I have read no study on; the risk factors (limited braking, narrow view) are practice knowledge.
Health-wise too: a low position can strain neck, back and hip. If it hurts, come out of it and have a fitting done. This guide explains and motivates, it does not replace a fitting or medical assessment.
Measure your position with a power meter, not with a mirror: ride five minutes each at two or three different torso angles at the same perceived effort and compare the watts. If you lose more than a few percent in the lower position, it is not faster for you, no matter how it looks. Ask yourself: can you still hold it in the last minute?
Low enough that you can still pedal hard and breathe. In a study with 19 riders power dropped with a lower torso, and a fully horizontal torso was never optimal [1][2]. The optimum depends on speed: above 46 km/h drag dominates, below 30 km/h more upright is better [2].
No. Below about 30 kilometres per hour a more upright position can be advantageous [2]. Also, in a study with nine men riders produced less power in the aero position than upright [3]. It is faster only if the drag gain exceeds the power loss.
No, aero bars on a normal road bike are enough to start. More important than the equipment is a position you can hold. This is practice knowledge, not a study finding.
It is individual. As an experience value you need several weeks until you can hold 20 to 30 minutes. This is not documented.
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