Modern Tetris has added a surprising number of additional gameplay features that make it play quite a bit differently from the classic versions. Some of those, like the hold queue and T-spin scoring, change the way the game is played, but others, like the hard drop, are basically just convenience features. One romhacker has taken the initiative to make NES Tetris a whole lot easier to return to with hard drop, and has even provided a detailed breakdown of how it works.
If you just want to play with the modification, you can download developer Stephen Sherratt’s IPS patch from Github and directly apply it to your own Tetris NES ROM. There are two versions of the patch, one of which adds a ghostly outline of the current piece so that you’ll know exactly where the blocks will drop – just like in modern Tetris – and one which makes you do some guesswork.
What’s really impressive here is that Sherratt wrote an extensive blog detailing the process of building this hack. He used a pair of NES emulators – one of which he developed himself – to observe the game’s code, and isolate the bits necessary to both create the ghost piece and add the hard drop. It’s too much to recap here, but the full breakdown is darn near readable even if you (like me) have a kindergartner’s grasp of programming.