Accel 2 click carries ST’s LIS3DSH 3-axis accelerometer.
Let's zoom inside the LIS3DSH chip, Discovery-channel style. Inside it are these microscopic suspended silicon structures that are attached to a substrate by a few anchors and are therefore free to move in the direction of acceleration. This causes an imbalance in capaticance, and that’s how the chip digitally quantifies its own motion.
Those X, Y, Z values are communicated to the target board MCU either through mikroBUS™ SPI or I2C interfaces. You determine which one it is by soldering the onboard jumpers in the appropriate position.
But the LIS3DSH chip can do more. Having two state machines built-in, it can send interrupt signals when it recognizes user-defined motion patterns (shaking, double shaking, turning upside down, things like that). Thus, it’s great for designing motion-based user interfaces.
It also integrates a FIFO buffer (first in, first out), which means that the target board MCU doesn’t have to constantly read data from the sensor in real time — thus lowering power consumption.