You're thinking of a delay unit, reverb effects ain't nothing new either, it goes way back to the days of Di
ck Dale... The delay unit repeats certain notes in a programmed pattern... However, delay is rarely done using a pedal, they are often produced through delay units plugged into an amp... The Edge for example produces his effects by plugging his guitar into an A/B box that splits into two large delay units plugged into two different sets of amps by using equipment that he designed himself, it creates his trademark "echo" sound where he can create lush, complex guitar harmonies (both on stage and in the studio) by only playing a few notes... David Gilmour has used a similar technique a couple of times, as well as a few others
But i think you are just jumping to conclusions, i'm
very sure Yngwie dosen't use reverb or delay effects, at least not most of the time, how can you say you know for sure what kind of pedal it was and what it's actual purpose is, all by just looking at it?... Most types of pedals look pretty much the same, but they can have various purposes, Yngwie dosen't hide what kind of equipment he uses on stage... He uses DOD 250 and DOD YJM308 overdrive pedals excessively, thats nothing his fans don't know, and it's more than likely that was the kind of pedal you saw him use... And like i said that kind of effect you discribed is rarely done on pedals anyway (though mine has a built in delay effect), most professional guitarists use standard delay units instead, if Yngwie wanted delay, then he would most likely use a unit instead of a pedal, because it's much easier than using a pedal... He may appear like he isn't playing many notes, but thats mainly because of his scalloped frets, which make his strings more touch sensitive, like those on a chapman stick, if you ever watched Tony Levin or other chapman stick players live, they also seem to be playing less notes than what they are actually producing, Levin dosen't even use pedals or units... Scalloped frets give players the appearance of playing less notes than what their instrument actually produces, Yngwie also uses a special downstroke/upstroke picking technique with his fingers, which may contribute to the illusion as well.
Check this out!
^ Yngwies stash.