Blipping - Blipping consisted in modifying the ignition sequence (not stopping a number of cylinders from functioning) , and consequently reducing the overall engine moment and therefore the engine expressed power.
However this was true when you were on ground with the engine running, while, in air, the situation was somehow more complex, as blipping did not cut the ignition, thence the bloc engine- propeller went on gearing. As in some situations, the propeller moment could be superior to the engine moment, and the propeller being bolted to the engine, the engine moment was then "dictated" by the propeller. In these cases, the effect of the blipping was felt only when the propeller moment decreased under the engine moment value. In other terms, you needed first to put your aircraft's nose up to reduce your rpm under 1200/1300rpm, then begin blipping. If you needed to reduce engine power/speed fast and for sure, the best way, as advised in ww1 manuals, was to cut ignition, something that would stop the engine and the propeller from gearing, and glide to earth.
In Il2 all this can be done rather straightforward, as Il2 uses energy moments (engine moment and propeller moment) as a basis for the engine part of the flight model - and not power, something that, interestingly, was heavily criticized in the past, as it is a source of error if the propellor's reductor value is different from 1 - but with rotary engine, where the propeller is bolted to the engine hub and rotating together with the engine, this reductor value is always 1.
So the blipping in the Gnome engine enables to reduce the expressed power (by modifying the engine moment as in historical reality) from 1200rpm to 1000 rpm then to 800 rpm, then to 0, cutting ignition. As in real blipping, so long you have not cut the ignition you can "blip back" from 800 to 1000 rpm or from 100 to 1200 rpm without need to restart the engine. Remember that restarting the engine in mid-air demands a minimal remaining speed and altitude to be able to do so.
(note: Later in war, the 160hp Gnome 9N has up to 5 different possibilities of blip position (0, 200 rpm, 400 rpm, 800rpm, 1000 rpm, 1200 rpm), however it has also a throttle allowing it to pass from 600 to 1200 rpm, so the blip was used, if necessary, only at start up, taxi, and landing. )
In a dive, shutting off the fuel was still a valid practice, even with later rotaries, as opposed to blipping. .