Baptisms are performed by a living person "on behalf of" a dead person. No one gets possessed or anything. 
 
Many people do report feeling a very close kinship, almost a presence, with the ancestors for whom they are being baptized.  
 
At the end of the day, there is no way to know whether or not the person has accepted the ordinance performed on their behalf.

This is from an article that President Hinckley wrote: 
 
"Through living proxies who stand in behalf of the dead, the same ordinances are available to those who have passed from mortality. In the spirit world these same individuals are then free to accept or reject those earthly ordinances performed for them, including baptism, marriage, and the sealing of family relationships. There's no compulsion in the work of the Lord, but there must be opportunity." 
 
President Gordon B. Hickley, Article "Why these Temples?" 
 
They are performed just as you would a normal baptism, just someone else is baptized for them "in their behalf."  
 
It is up to the person who has died, and now on the "other side," to have the chance to accept or reject that baptism that was performed "in their behalf." They aren't "forced" into the Church by that baptism or anything like that. They still have their free agency to accept the gospel.

In many Christian religions you are required to be a member here on earth to go to heaven but we believe that after death you have the chance to accept the gospel, which is where baptisms for the dead come in

All temple work for the deceased is perfumed by proxy, a living person stands in the place of the deceased and receives the ordinance on behalf of the person who is dead.  
 
This process though does not make anyone accept the ordinances performed it only gives them the opportunity to accept the ordinance perform on there behalf if they wish to do so.  
 
In the end free will is most sacred, the ordinances are performed for all because Christ commands us to ensure that all have the ability to accept the full blessings of the Gospel of Jesus Christ. If not God would cease to exist because he would then be a respecter of persons and of not said the truth when he declare he respects no man.

Heavenly Father loves all of us, even though we make mistakes, and things we can repent. The LDS Church has the TRUE gospel. In this church everything makes sense and there is no doubt in my mind that this church is true. The Temple is a sacred place, only LDS people can go in there because it is so special. There are such things as Temple Open houses, you can go inside then.

Now that the ceremonies have been filmed and are all over the internet they are no longer a mystery. How will missionaries deal with investigators questions concerning the temple when anyone can go online and see them for themselves?

Linda, 
 
As long as I can remember the Church has taught the temple is not about keeping it secret - rather it's about keeping it sacred. 
 
Just as the Savior taught not to trivialize sacred things (analogy of pearls before swine) - the ordinances of the temple are to be treated as very sacred. Just as a husband and wife don't talk about personal things - our personal relationship and the very special and tender spiritual feelings and relationship we have with our Heavenly Father should be treated sacredly. 
 
It's always been interesting to me to observe how many people seem they broadcast to the world miracles or special spiritual things that happen to them. On the other hand - most LDS members are hesitant to share very personal and spiritual things with other people - unless the situation is just right. We are taught to be very careful with how we treat the divine. It really is about treating sacred things with sacred care. 
 
So you will not see many mormons - if any - talking about the sacred things in the temple even though it is on the internet. (To be honest - most of the things in the temple has been available in some form or another (book, video, etc.) for decades. And that hasn't changed how Mormons treat our sacred temple covenants.

Jett, 2 things struck me in your reply. First you said husbands and wives don't talk about personal things. I find that very odd. Wouldn't your spouse be the first person that you would want to share everything with? And second, I realize that the temple ceremonies have been out there for awhile but I highly doubt that investigators would be encouraged to go online and take a look.  
 
Linda

Linda, 
 
I am not saying that husbands and wives don't share things with each other. I am saying that personal, private things between a husband and wife are not shared with other people. Of course couples talk with each other. But personal and private things between them should not be broadcast to the world. 
 
2 - Correct. We would not encourage them to go online and look.