Kigoroshi is a woodworking technique for removing gaps in notched woodworking joints. Japanese Joinery shared an amazing video detailing the process! Free and Handy shared this definition of Kigorishi:
Kigoroshi (木殺し) – a woodworking technique used for mortise and tenon joints (or other notched joints) to aid in snug fits. By hitting the appropriate part of a tenon with the rounded end of a hammer, hard enough to compress the part in question but not too hard that you crack or otherwise damage it irreparably, you can join the mortise and tenon with ease. Prior to joining, wipe or splash some water into the mortise; this will cause the compressed wood to expand to its original dimensions.