"Wave" is the termed name because the items drop in "waves" i.e., all at once over a short period.
How it works? Don't really want to post this but there's so much mis-information around...
To elaborate on the timing loop;
Obviously the segments for ammo, shrapnel, and basic oil are 100% of the loop. Then you have "windows" where items can appear, i.e. with the mushrooms, they have a 20minute window around the hour of time (10:50-11:10 for example).
To also get the item, you have to hit the correct value of loot (you can't get a GNA, for example, in a loot value lower than 5ped).
The second part, why unL's drop quickly after VUs/events starting (and something which I find
far more damaging than the principle of timing drops (apart from the predictability) is that as soon as the item cap is raised, those items are available to loot. So if in the picture example there was 1 mod fap added, it would not take very long at all (on a popular mob, just 1 run through the timing loop) until the mod fap was looted.
This is where MA should concentrate on alterations. But I'm told I'm wrong by many others so...