'Weak' and 'focused' may seem like an oxymoron, but you have to look at the terms from a different perspective. When we say 'focused' we mean you - the angler - and your concentration. It is a measure of how much you narrow your eyes, quell your breath, and attribute all of your senses to the timeless art of poking hooks into fish. This potion increases those faculties. You become so focused that nothing else in the world matters.
And by 'weak' we mean that the potion is a little bit rubbish at doing all of the above.