transistor drums is a 808 / 909 style drum machine.
It uses different kinds of sound generating: the kick drum works analogue-style, the snare drum works hybrid analogue-style/sampling, the clap is granular sampled and the rest of the drums are built with samples.
Velocity affects not only volume - it also has an influence on the EQ in the background, analog parameters and also (minimally) to the envelope. Also built in is a microtuning random generator which generates for every drum beat a tiny difference mainly on pitch but also on other parameters to generate a more sensitive pseudo-realistic sound. A patch random button is included to help in creative crisis.