Topic little question
As with any key in starcraft, or anywhere for that matter, if you hold the key down, it has the same impact as tapping the key multiple times.
The same thing can happen with spellcasting. If you hold down "T" or "F", with infestors, you'll use infested terran with each left click or fungal with each left click depending on the key.
With HT you'd get feedback or storm at a really fast rate. With Sentries, you can place FF without deselecting your sentries by spam tapping the F key.