From Sonic Retro
The Drop Dash is a technique used by Sonic in Sonic Mania and by Classic Sonic in Sonic Forces, which is best described as a "midair Spin Dash". By pressing and holding any jump button in midair after jumping, Sonic will charge up the move, and will instantly dash off in a short burst upon landing. However, the move cannot be performed if Sonic takes damage before landing, or if an elemental Shield is active. If the move is executed by Super Sonic, a mini-earthquake is set off and the boost will be greater.
By collecting enough medals in Sonic Mania's Bonus Stage, this move can be replaced with the Super Peel-Out or the Insta-Shield by accessing the Secrets menu when starting a "No Save" game originally, but in v1.04, the other moves can now be played in a saved game. The player can also use all moves at once by putting in a cheat code in the secret level select.