Can I stop concentrating on a spell then start concentrating on it again before it expires? For example, if the spell lasts for 2 rounds after I cease concentrating, can I pick it up again before it ends?
If a spell lists "concentration" as its duration, then no, as soon as you stop concentrating on it, it ends (or continues on for the listed duration after you quit concentrating and THEN ends). You cannot pick it up again.