The game intentionally delays the start of your attacks by 1 frame in order to properly add leniency to multi-button moves (such as 2H+P+K aka the Side Step command). But this is not always true. This delay gets "bypassed" in 2 common scenarios:
- the actual attack blocking function (H) activates 1 frame earlier than the blocking animation does, which is why inputting a frame perfect Hold at an incoming attack will Block it instead of Holding it;
- wake up actions executed while lying on the floor start 1 frame earlier compared to standing. Here's a little demonstration with P+K that shows the difference between the two:
For reference, the Street Fighter series handles it differently: no delay is added, but 1 frame of leniency still exists. Nearly every move can be cancelled on the 2nd frame if you immediately override them with other moves that have higher priority. This can be exploited to perform moves that move you forward more (e.g. kara throw, kara shoryuken).
Tekken has no such mechanic, and it shows. Executing moves that need multiple buttons such as a simple LP+RP without the help of macros or buffer tricks must be done at the same exact frame, there is no leniency.
Standing moves happen 1 frame later for accessibility reasons. Not true for grounded moves since they happen 1 frame earlier than that. Blocking also happens 1 frame earlier.