Alright, it's about time I actually made a post about tech chasing. I'm sure lots of people want one, especially because it's *not* just applicable to sheik. Most characters have an application for tech chasing. Without further ado, let's begin...
First, some frame data. Tech in place lasts 26 frames, and tech rolls last 40 frames. This does not change based on the character. Grab hits on frame 7, which means to hit tech in place you can grab at the latest by frame 19. The other limiting factor is human reaction time, and I don't think any player is realistically capable of reacting before frame 18 on a very consistent basis. So to grab tech in place you need to be positioned close enough to their landing, and input grab on frame 18 or 19. The tech rolls are easier to hit since they take longer, provided that your character has a way to close the gap; in sheik's case I can boost grab, with marth and fox I can just dash into a JC grab, with peach I can dash attack etc. As the 20gx guys have mentioned, as long as you react by the frame 18 I just mentioned, falcon can even knee/stomp the tech roll away and behind, respectively.
There are many ways to tech chase effectively, and my best guess is that the more of them you are aware of, the better. At the very least, it can't hurt. So let's start by looking at the animations:
All 3 of fox's tech animations in real time: http://imgur.com/a/Onsab
Fox's forward tech: http://imgur.com/gallery/sX7Ed
Fox's backwards tech: http://imgur.com/gallery/Qq6Pv/
Fox's tech in place: http://imgur.com/gallery/1E3eQ
Miss tech is such a different animation than the others that I don't think you need to study images to recognize it. The character has no tech animation and there is a big green flash. I won't go into detail about how to cover it, because it is fairly character specific (both in terms of the character tech chasing, and the character being tech chased). At a later time I might write about it, or I could answer individual questions about it if anyone has them.
Note that in these images there's two major things you can pay attention to: 1) the animation/physical contortion of fox's body 2) the white and yellow lines that signify the tech. Studying these can be useful, and I am fairly certain that I at the very least subconsciously am aware of both of them.
Outside of using the actual animations, the other important way to approach tech chasing is by learning the timing/rhythm of the tech chase. So the key here is that after they hit the ground, you make a decision on frame 18 of their tech. A huge factor is that when you are watching a character tech and you are reacting, you just check if their character model is still in the same place they landed. If their character model hasn't changed physical locations during your reaction window *and* you see a tech animation at all, you know they must have teched in place. If by the end of your reaction window you realize the character is no where near their initial landing spot, you know it must have been some sort of tech roll. If you keep this in mind, then the real trick to tech chasing is learning the timing for what frame you need to make a decision.
One way of testing this is to go into 20xx's develop mode, and press y+down on dpad to turn on the animation counter. When you grab a fox with sheik let's say, down throw him and let him tech. Press start on the frame you think you need to react and would normally input grab for tech in place; if you are pausing on the 18th or 19th frame of the tech, you've found the timing! (in 20xx, this will be displayed as 17 and 18 because the game starts counting from 0, not 1)
The goal is to be able to consistently react on the correct frame. In that sense, tech chasing is as much of a rhythmic challenge as it is a reactionary challenge. Something I've been considering lately after talking to Dizzkidboogie about wobbling is that music/a metronome could also be used to help with tech chasing! If you think of the frame they hit the ground as the first "beat" and frame 18 (the point in time you need to make a decision) as the second "beat" then that means there is a beat every 18 frames. 18/60 = 3/10, so a beat every 3/10 of a second, which is 10 beats every 3 seconds -->200 beats per minute (the same as wobbling, funnily enough).
For reference: https://www.youtube.com/watch?v=bgk8AfY1u40
Alternatively, music at 200 bpm: https://www.youtube.com/watch?v=s7pF6_PeamU
While tech chasing in 20xx, I experimented with having the metronome on during tech chasing. I discovered that the times I didn't grab the tech in place, I was clearly too slow with respect to the beat. When I focused harder on just matching the tempo no matter what, I found myself hitting the options more consistently. Of course there is some bias because I already know how to tech chase, but I can see this being a very useful tool for learning the rhythm of tech chasing. Of course, it is generally unlikely that the music/metronome's beat will match up perfectly with when the character hits the ground; ideally you have a good enough intrinsic sense of the beat that you can sort of mentally shift it appropriately.
So let's say you have the rhythm down, you recognize the animations, the whole shebang...is there anything else? Absolutely! It might depend on your character, but let's take sheik's down throw as an example. One of the things that takes attention away from purely focusing on the tech animations is their DI on the actual throw. The faster you react to the DI they did on the throw, the more time you can spend focusing on the actual tech reaction. If they DI away you should walk towards their landing, if they land in front of you directly you should do no DI, and if they are going to land behind you then you should turn around. With various slight DIs this can be ambiguous, but enough practice will help you recognize which side they will land on better and better. Improving your reaction to the DI on down throw as sheik is almost as important as being good at recognizing the techs; as I said, it takes up valuable mental focus for the reaction.
The last thing I will say is that my experience with tech chasing leads me to believe that discipline is one of the most important parts of this. People ask me how I got so good at it (even though I drop like half my tech chases in tournament LOL) or whether or not I would tech chase in a last stock situation where if I drop it I will die...the answer is yes, I will. The reason I'm better at it than everyone else is because I'm more committed to it; I'm willing to lose trying my best to do it, in hopes that I will eventually not mess it up. I've probably been tech in place shined more times than anyone in the entire history of the game, and that's not an exaggeration.
Have fun everyone, and happy tech chasing!