A tick throw involves attacking, and aiming for a throw (normal or command) just as the opponent is made more conscious of blocking.
Usually, a tick throw setup involves touching the opponent with some sort of tap or a jump attack and just as they unfreeze from the resulting stun (blockstun included), having a throw ready to take them. Because (in general cases) the opponent cannot be thrown during their stun, but by definition they cannot do anything during their stun, the throw is going to be more likely to work due to the tick.
The term “tick” in English lingo derives from the sound effect clip that plays when the initial attack is blocked (especially the sound from old versions ofSFII, where it is clearly a “tick” instead of the bone-crunching smacking whacking sounds given off by hits, and where normal throws did lots of damage).
For characters that don’t have (good) overhead moves or are bad at option pressing for some other reason, then to some extent, good tick throw technique can compensate for this lack to some extent.
Tick throws are generally weak to flailing. However, in series such as The King of Fighters where the buffer gives a longer window for command input acceptance, it doesn’t matter too much whether the flailing touches; afterwards, the throw can still start up, and still work.
In recent years, 2D fighting games now reduce the chance of a successful tick throw, through evasive measures like throw escapes, less punishable jumps, and flailing; however, in older games, the startup of throws was quick, and in many cases, there was no such thing as teching throws. Therefore, there was no simple and/or apparent way to evade the throw after the tick, and it was overwhelmingly effective, basically landing it in the zone of “cheap”.
However, it isn’t all that cheap compared to plenty of other things because the method of evasion is simple, and in fact, there are rare instances where AI will attempt it, too (in particular, Gōki and Ingrid in the Street Fighter series).