Measuring roughly 40 cm by 40 cm, the podruchnik takes the form of a small quilted cushion, often with patchwork decoration which varies from region to region. Its sole function is to keep the worshiper's hands and face clean during the frequent prostrations that occur in church services, as it is felt that the fingers used to make the Sign of the Cross must remain clean during prayers out of reverence. Additionally, one should have clean hands when handling prayer books, Holy Scripture, and when using the prayer rope. When making prostrations, the prayer rope, as well as the hands, is placed on the prayer rug, and not on the floor, so that there is no trampling on sacred symbols, and/or the cross that are on the bottom of most prayer ropes.
Lastly, when receiving prosphora one should have clean hands as this is bread is holy, and used for the Divine Liturgy. This bread is the remainder of the loaves from which the small particles were taken out to be used in the Liturgy for the Holy Lamb, and other commemorative particles, and therefore it has a special blessing, although it is not the Body of Christ.
The word podruchnik essentially means the thing under the hand or arm. Besides the Old Believer prayer rug, it also means “armrest,” “elbow rest,” “apprentice,” etc.
During church services, when prostrations are made, as the worshiper lowers his torso, he flips the podruchnik onto the ground in such a way that when he is kneeling and prostrate on the ground, the podruchnik protects his hands and forehead from the dirt and grime of the floor.
When not in use, they are stored at the side of the church, stacked up in such a manner that the dirty sides touch each other and the clean sides only touch other clean sides.
Symbolism:
Square - Earth
4 (triangles within a square) is the number of Evangelists.
5 (squares) - the number of wounds of Christ
7 (concentric squares) - the number of Councils and Sacraments.
9 (squares) - the number of Angelic ranks and Beatitudes.
12 (triangles: 4 squares one in the other) - the number of Apostles.
The prayer rug can also be "simple", i.e. not from triangles of different fabrics, but from one square piece. After all, its main purpose is not to remind the worshipers of the number of Ecumenical Councils, Gospels, etc.
A demonstration on how the prayer rug is used can be seen in the following video from 10:46 to 11:10:
https://yandex.com/video/preview/12059730243289812723
Without a prayer rug a person can be kept most clean with his or her hands with the following method:
The left hand is clenched into a fist, and placed on the floor with the edge of the palm towards the bottom; This fist is covered with the palm of the right hand, and the outside of the right hand is touched by the forehead. When making a prostration to the ground in this way, only the edge of the palm of the left hand gets dirty. At the same time, the palm and fingers of the left hand remain clean, and with them the right hand, with which the sign of the cross is overshadowed, as well as the forehead (forehead). This method is quite acceptable in cases when the worshiper has to make a prostration without being able to prepare for it in advance. 
The upper part of the prayer rug is often from a more expensive and higher quality material than the lower one. But, at the same time, the lower part, as a rule, is made of a denser material than the upper one. The upper part, in addition to beauty and symbolism, can also have pleasant to the touch functions. This is contrasted with the lower part, which has only practical functions, such as high wear resistance and moisture resistance. The inner (middle) part of the prayer rug is made of a padding material. This may include: felt, cotton, wool, fabric in several layers, and even horsehair. If necessary, the stuffing material is additionally sewn or quilted, so that when using the prayer rug, it always holds evenly over the entire area, and does not roll or knock off. These particular podruchniki are filled with sheep wool.
Sometimes, for convenience, a handle or a loop is sewn onto the front of the prayer rug, which is not conspicuous, and does not spoil the overall picture of the prayer rugs symbolism. This is done, first of all, for hygienic purposes, so that when picking up the prayer rug from the floor the faithful person does not to touch either the floor itself or its dirty side. Oftentimes, a small loop is made or sewn in one of the corners of the handy so that the handy can be hung on a hook. Some craftsmen even manage to sew pockets into prayer rug, which does not contradict tradition.
Due to each podruchnik being handmade, the exact color of each may vary from what is shown in the product image.