This is a really nice drum machine that utilizes DIP switches to control the drum beats. Those types of switches are hard to manipulate, but I imagine you could easily replace them with an easier to use 2 or 3 position toggle switch.
The two dip switches work in tandem so that each 1/8th note can have four possibilities (two bits) worth). If the position on the right switch is 0 and the position on the left switch is 0, then nothing is played. If the setup is 1 and 0, then a kick sample is played. If the setup is 0 and 1, then a snare sample is played. If the setup is 1 and 1, then a hi-hat sample is played.
More about the Arduino drum machine
More about the Physical drum loop