This is a detpack. They are timed explosives designed to take out anyone in a room in a single blast. Unfortunaely, I don't use them as often as I should. In case you haven't figured out, I'm pretty hung up on honour and skill in Team Fortress, and I actually consider the detpack to be a form of spam, more or less. It's just like a MIRV grenade; you put it down somewhere in the hopes of fragging someone. However, they can be useful in a clan match situation.
Setting a Detpack
Placing a detpack is very simple. There are three time lengths that you can set one for: 5 seconds, 20 seconds, and 50 seconds. The commands for setting a detpack are as follows:
+det5 - Set a detpack for 5 secondsYou should bind each one of those to a certain key (preferably keys that are close to one another on the keyboard). For example, for me the letter "J" is binded to +det5, "K" to +det20, and "L" to +det50. That way, the keys are right next to each other, and the timing on the detpacks increases as I move my fingers from right to left along those particular keys. It's just easier that way, as opposed to having various keys all over the board in random places binded to detpack commands. To actually lay the detpack, simply press and hold the key you have chosen until you see the words "Detpack Set!" appear at the top of the screen. Then run like hell (especially if you set it for 5 seconds)!
+det20 - Set a detpack for 20 seconds
+det50 - Set a detpack for 50 seconds
There is one major misconception about detpacks, and that is that you can customize the time that you set one for. By this, I mean that there are those that believe you can set a detpack to go off in 30 seconds by using the command +det30. Nope, sorry - it doesn't work like that; you only have the three times mentioned above to choose from.
Uses for Detpacks
Now, I know people who swear by detpacks, and don't go through a single game of TF without using a detpack for each life they get. I just don't do it like that, and if you didn't read my reasons for doing so, read the first paragraph of this page.
The only time when I use them is on defense. For example, say that I'm defending a strategically important point in my base (such as the ramp room or the basement in 2fort5). After a battle in which I've been hurt and am forced to retreat to my resupply station in order to get health and arms, I set a detpack roughly in the place where I am standing on Defense, and then proceed to the resupply. I really only use this strategy in clan matches, and aside from this, I never use detpacks.
I know and have seen many players use detpacks as an "offensive" weapon. By this, I mean that they set detpacks in the enemy base itself, such as in the main lobby in 2fort5, or the upstairs area outside the resupply. I myself don't do this because, as I have said before, I consider it more or less a form of spam. However, I don't think badly of anyone who does do something like this. The time it takes to set a detpack in the enemy base can often put you in danger. As you're setting it, a Soldier could easily come up from somewhere and kill you in two shots (or at least severly hurt you). If you're in the enemy base, there are things that will help your team out a lot more than setting a detpack, such as getting the enemy flag, for example.
One more thing to keep in mind about detpacks is that they don't discriminate. By this I mean that they'll damage players in the same manner as a convential weapon. If you fire a grenade at a team mate it will (depending on the server settings) do one of a few things. It could do nothing, hurt their armour, or it could hurt them as if they were a member of the enemy team (there are other possibilities, but these are the major teamplay settings used on servers). So keep in mind that, when your detpack goes off, it will effect everyone around it, including your team mates. A lot of people find loosing their armour or their lives at the hands of their own team mates to be quite annoying.