The following is from Daf-Background to Kidushin 22b. I hope that it is helpful to you. (Regarding the underlying, fundamental mechanism of the Kinyan, this topic is the subject of much discussion among the Acharonim and is beyond the scope of this forum.)

Y. Shaw

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(a) When a person buys an object, he must make a Ma'aseh Kinyan, a formal Halachically-binding act denoting his acquisition of the object, in order for the sale to be irrevocably binding. Depending on what object one is acquiring, different Kinyanim are used, as follows.

(b) The forms of Kinyan that may be used for the acquisition of Metaltelin (mobile items) are:

1. Hagbahah, i.e. lifting the item;

2. Meshichah, i.e. pulling the item or causing it to move;

3. Mesirah, i.e. handing over the reigns of an animal or the tie lines of a boat, or a bill of debt (Shtar Chov);

4. Chatzer, i.e. bringing the item into one's private domain;

5. Chalipin (exchange or barter), i.e. taking another object to demonstrate one's consent for the Kinyan (or for an agreement).

6. Agav, i.e. acquiring the moveable object automatically upon the acquisition of land.

(c) The forms of Kinyan that may be used for the acquisition of land are:

1. Kesef, i.e. paying at least a Perutah's worth of money for the land;

2. Shtar, i.e. receiving a legal document containing the details of the sale;

3. Chazakah, i.e. performing an act that is normally performed only by an owner;

4. Chalipin (as mentioned above, b:5)

(d) The source for Kinyan Chalipin in the Torah is from the verse in Ruth 4:7-8, in which this Kinyan is used by Bo'az. Although Kinyan Chalipin may be accomplished through the barter of two equally-valued items, it is normally no more than a symbolic act in which an object of little value is given over in order to represent the acquisition of an object of value. For example, Chalipin may be accomplished by taking possession momentarily of a scarf or piece of cloth (Sudar) that belongs to the other party in order to make a Kinyan on another object that is being transferred. For this reason, Chalipin is also known as "Kinyan Sudar." (The other object is not simply handed over to the buyer to make the Kinyan with Meshichah either because it is not present or because it is too large or it is unfeasible to hand it over, for example in the case of the transfer of land.)

(e) The Gemara records a Machlokes among the Amora'im as to whether the Sudar used in the symbolic Kinyan is given by the buyer (in exchange for the object that is being acquired) or by the seller ( along with the object that is being acquired). The Halachic ruling is that the buyer gives the Sudar, and in return he acquires the object that is being transferred (Bava Metzi'a 47b).