Belt Method (also known as Equator or Ring) is a middle-layer-first method for the 3x3 proposed by Denton Holder. It and other middle-layer-first variants have been proposed a number of times, e.g. Ring Method in 2004 by Kyle Bryant.
1. Solve the E-Slice
2. Separate the remaining pieces into their respective layers. The corners and the edges are usually done separately, but the order varies. This can be done intuitively.
3. Orient the U and D layer pieces. Doing this step in one look requires far too many algorithms, so it is normally done by first orienting the U layer, and then the D layer. OLL (57 algorithms) can be used for this, though a parity algorithm is needed.
Corner Orientation Parity (To be repeated if it does not work): R U' R' U' R U R' U2 R U' R'
Edge Orientation Parity (At least one unoriented edge in the M Layer): M' U M
4. Permute U and D layer pieces. The one look version takes over 300 cases, so, once again, two look is much more practical. You first correct the parity error (which occurs 50% of the time), and then you use PLL (21 algorithms) on both layers to finish the cube.
PLL Parity- M2 U2 M2
1. Solve the E slice intuitively
2. Insert and orient edges. Use M U2 M' if the bottom side is on the top part of the edge. If the bottom side color is on the front of the edge, use R' M' R M
3. Insert and seperate corners with R U R' U' R U R' U' R U R'. If after inserting an edge an oll isnt possible, use F2L 39 and F2L 40.
4. Do oll and pll. If you 2 edges switched, use M2 U2 M2
5. Do oll and pll on the top side.
There are many variants to the method described above. Instead of seperating, orienting and then permuting, you could change the order around. You can do orientation, seperation and then permutation. Another slightly different approach is to orient all edges, THEN solve the belt and continue on.
Overall, Belt method has a reasonably large move count and is not as fast as other speed methods (CFOP, Roux, Petrus, CF etc.) However, it is a lot of fun and has been used to achieve sub-30.