How do crystals slip is a fundamental question in crystal plasticity that has long puzzled physicists and materials scientists alike. The early experimental work of Elam, Taylor and others in 1920s established convincingly that single crystals of metals under stress develop large slips along certain crystallographic directions. By 1930s, the role of dislocations in crystal deformation was already established. The observation by transmission electron microscopy in 1950s furthered the study of dislocations, their characters, microstructure and their interactions that formed a foundation of modern materials science. However, relating microscopic dislocation motion to mesoscopic mechanic properties remains extremely challenging. Basic phenomena, such as work hardening and persistent slip bands, remain unexplained. Recent research efforts have emphasized intermittency in crystal plasticity related to dislocation avalanches, but the mechanism is again a mystery. Here, we report our exploration of the mechanism of large crystal slips using a combination of mechanic sensor and direct TEM imaging. We show that a broad range of dislocation motions from nm/s to mm/s can be measured in compressed nanopillars. Using our technique, we followed the development of dislocation avalanches in a multi-principle element alloy, Al0.1CoCrFeNi. Results show that the avalanches starts with dislocation accumulations and the formation of dislocation bands. Dislocation pileups form in front of the dislocation bands, whose giveaway trigs the avalanche, like the opening of a floodgate. The size of slips ranges from few to 102 nm, with the power-law distribution similar to earthquakes. Thus, our study identifies the dislocation interaction mechanism for large crystal slips, and provides critical insights into the deformation of the alloy.