Glitch Rush, in d2r, that means rush from normal to hell game using a "bumper" to bypass Baal in each difficulty.
Bumper has to be level 20+ in norm and ancients done and Baal quest not done yet. If conditions are met, killing Baal with Bumper allows leechers to reach nightmare.
Same for nightmare to hell with a bumper level 40+ and ancients done.
Grush is also compared to Crush which is "Classic Rush", rushing in classic mod then switch to expansion to bypass ancients / Baal level restriction.