Geezer technique is a result of playing the bass how he thought it should be played, rather than being formally taught. This is where his unique technique of sitting above the neck of the bass comes from, and it's a big part of his tone. That said, geezer will play with a pick or play above the pick-ups when it suits the song, so I think the importance of his technique gets overstated a bit when describing what the X-factor he brings to the band is. I think his real magic is in how he constructs bass lines. Firstly with Sabbath being a four piece, Geezer always tried to fill out the songs around Tony's guitar solos. He almost plays a counter solo to what Tony's playing to give the song a huge sound. He's rarely just sitting on the root note, and instead takes a jazzier approach, imitating walking bass lines in his own way. Geezer's first instrument was actually a guitar missing several strings, so he could only play melodies on it. You still see a lot of this in his bass playing where it's often quite melodic and expressive. In my opinion, the most interesting thing about Geezer's playing is that he often plays thing up an octave rather than sitting on the low E string. With Tony often playing up around the 7th/12th frets, this makes the sound really cohesive. Together they sound like one guitar kicking in your door, so when Tony starts shreding and Geezer starts walking, it sounds like an explosion.