I feel that a bit about the Half-Life ritual of "Humping the Pole" would be a good supplement to rookkey2’s excellent write-up.
Many newbies and even experienced players are ignorant of this long-standing dustbowl tradition. Its origins are obscure, but certainly lie somewhere in the pre-round boredom that so commonly afflicts the attacking team. As we all know, idle hands are the devil's playground.
To give the reader an idea of the time frame I'm talking about, there is a period of about 90 seconds before the gates open and let the attackers through. For an attacking player, chomping at the bit, this is an eternity. For the defending player, this is barely enough time to get his sentry gun set up, or plant his detpack. As you can see, this pre-round delay is a necessary evil.
Naturally, the attackers can put this time to good use by tweaking various settings, changing their class, or discussing strategy. Of course, most players on public servers want nothing to do with any of that; so, "Humping the Pole" was born.
The activity, itself, takes place just before the third and final stage of the map. Outside the respawn there is a signpost that says, "Capture Point", and has an arrow indicating the appropriate direction. This is the pole that will be humped. There are similar signs before the first two sections of the map, but they are not humpable.
To "Hump the Pole", first stand next to the pole facing it.
Make sure that the sign is above your head.
Now, jump up and down as rapidly as you can.
If you are doing it properly your head will be hitting the sign above you, causing you to rebound toward the ground.
To anyone watching, it looks as if your character is trying to molest the pole. The defenders, of course, are unable to see any of this, but they will hear an awful lot of grunting as you jump up and down, and are sure to know what's going on.
Humping the pole brings good luck to you and your team; so do it often!