In the may/june 1996 issue of NOSTalgia, the NOST bulletin, Kerry Handscomb writes about this Shogi variant which he calls Microshogi.
He has played the game in 1981, and assumes that it was invented by the great shogi player Oyama Yasuharu. (Oyama Yasaharu died several years ago, at an age of approximately 60 years. He was a top level shogi player until he died. He is the 15th "Eisei Meijin" (Eternal champion), and he was the president of Japanese shogi association.)
Wooden and plastic sets are (most probably) commercially sold in Japan, and were used in a promotional campaign of a firm. The rule sheet for the game calls it Five-minute Poppy Shogi, in Japanese.
The game is played on a board with five rows and four columns, i.e., only 20 squares. Each player has only five pieces. Using a nice promotion rule, still most of the Shogi pieces can play a role in this game (with only the dragon king and dragon horse - promoted rook and bishop omitted).
At the start of the game, each player has a king, a bishop, a golden general, a silver general, and a pawn. The opening setup is as follows:
King d1, Bishop c1, Golden General b1, Silver General a1, Pawn d2.
King a5, Bishop b5, Golden General c5, Silver General d5, Pawn a4.
In this game, pieces do not promote by reaching a certain rank, but promote when they take another piece. Each time a piece takes a piece of the opponent, it is reversed. At the other side of the piece, another piece is shown:
Promotion (which is in many cases more a degradation) is obligatory: every time a piece takes another piece, it is reversed. For instance, when a golden general takes a piece, it becomes a rook. When this rook takes a piece, it becomes again a golden general, etc.
All pieces move in the same way as in Shogi (except that they do not promote in the same way.)
These promotion rules do not hold for kings, i.e., when a king takes a piece, it just stays a king.
Pieces taken from the opponent are held in reserve, and can be dropped on empty squares on the board, as in Shogi. There are some differences in the rules: one may put any side of the piece up when dropping a piece, e.g., when one has taken a silver general, one can decide to drop the piece as lance or as silver general. One may drop a piece on a square from where it can no longer move. Pawns may be dropped on columns that already contain another pawn of the player (so, this restriction of Shogi is not in this game). It is allowed to give mate with a pawn-drop (as with a drop with any other piece).
Object is to take the opponents king. Giving perpetual check is forbidden.