Is it possible to fly 'in neutral'?
Much like how a gas pedal controls the fuel input of a car's engine, so do the thrust levers on jet-liners. The coasting equivalent is idle thrust. The engine keeps running, but with minimal fuel flow.

The minimum fuel flow is not fixed as it is a function of altitude, airspeed, and temperature. Nowadays it is typically managed by the FADEC, so the pilot (or auto flight system) can pull the levers all the way to idle and not risk flaming out.

Idle thrust in this answer refers to that minimum thrust setting that would not cause a flame-out.

An ideal descent (and its planning) does just that. Descend from cruise all the way to lining up with the runway using idle thrust.

Hitting unexpected headwind, or being asked to slow the rate of descent, would require higher than idle thrust. In the car/slope analogy, that would be the equivalent of changing the slope to a shallower one (planes, unlike cars, need a minimum forward speed).

Most jet engines also supply air for air conditioning and pressurization – it's called bleed air. On a plane like the Concorde that cruised at almost double the altitude of the common jet-liner where the air is even thinner, idling at 55,000 feet or so was not an option because of the impact it would have on the pressurization. The engines needed to run at higher than idle to provide this bleed air.


(Source: ITVV Concorde) In-flight deployment of the Concorde's reversers. Highlighted is the secondary nozzles position indicators – the nozzles double as reverse buckets.

If the Concorde flight crew wanted to increase the rate of descent, they were allowed to deploy the reversers of the inboard engines (#2 and #3) in-flight – this was done from the flight engineer's panel (shown above). This way the thrust is redirected, but the engines keep running at the required RPM for this bleed air. (According to the Type Certificate, this was limited to 4 minutes. That's about 8,000–12,000 feet of descent.)