The primary combat strategy of Tai Ji Quan can be summed up in the phrase "Entice (the opponent) to advance, (cause the opponent to) fall into emptiness, unite (with the opponent) then throw (the opponent) out" [Yin jin, luo kong, he ji chu]. Enticing the opponent to advance (advance refers to the opponent's aggressive forward momentum) can be as simple as standing in front, presenting an open target or launching a preemptive attack designed to draw a reaction. Enticing the opponent into aggressive forward momentum has several advantages...
Finally, enticing an opponent into aggressive forward motion locks his mentality into the attack mode. With committed focus on attacking, the opponent will be slow in changing to the defensive mind set as the Tai Ji Quan fighter counterattacks. The opponent's reaction time is delayed, further increasing the counterattacks odds of success; this allows the Tai Ji Quan fighter to "leave after yet arrive first..."
Stick and Adhere refer to connecting with the opponent in a soft and nonconfrontational manner and maintaining this connection as you both move (blocking an opponent's incoming force inevitably results in the opponent being knocked away. This makes it impossible to join with the opponent and one is doomed to remaining double weighted).
Most often, the Tai Ji Quan fighter will seek to stick to and control the opponent's center through contact with his arms and/or upper torso, using these regions as handles to the opponent's center.