timeboxing means allocating certain amount of time to work on some specific task. while working it's easy to get sidetracked from the original task, so it's important to keep asking yourself, "what am i working on now", "what am i trying to achieve by working on it" "what's my approach" and "by working on it, am i making progress on the task".
don't use a wall-clock for time tracking, it's a distraction, instead use a timer application like alarm-clock-applet which tells you "how much time is left" in this timebox. when i use a timer, i feel as though time doesn't exist and it's just me in a box. clock on the other hand introduces an overhead about what time it is now and all the implications that awareness brings about.