The hanging pyx is a suspended form of tabernacle, or place of reservation in other words, for the Blessed Sacrament.
Archdale King in, Eucharistic Reservation in the Western Church suggests that it was a "very general" (though not universal) form of Eucharistic reservation through England, Scotland and France during the middle ages -- and in France, later still according to Peter F. Anson.
Of its characteristics, ornamentation and function, King has this to say:
The usual method for fixing the pyx was for a crane or pulley to be so arranged over the altar as to permit of the ready raising or lowering of the pyx, which was suspended by a cord or chain attached to a ring on its top. Above the pyx was hung the canopy, a circular tent-like construction formed of some costly fabric, which was generally attached to a ring and ornamental crown of metal. The pyx itself was veiled in a pyx cloth, which often had the form of a square napkin, with a hole in the middle, through which the suspending cord passed, and weighted tassles at the four corners which kept it down close by the pyx.
One extant mediaeval example of the pyx cloth that covered the pyx is to be found, coming from the Hessett Church in Suffolk, England:
(Image source: Allan Barton)
As well, above the pyx, King mentions that a circular tent-like canopy was also suspended. This construction might be seen in various mediaeval manuscripts. Here is an example:
(Image source: Allan Barton)
Peter F. Anson's book, Churches: Their Plan and Furnishing shows a number of examples of some different forms of hanging pyxes.
Here are three photographic examples of three of the styles illustrated above by Anson:
A gothic revival triple crown pyx, found in Grosvenor Chapel in Mayfair, London, by Sir Ninian Comper. (Image by Lawrence Lew, OP) You can see the tassled pyx cloth.
The dove pyx which Archdale King suggests were most commonly found in France, but not so much England or Scotland
(Image source: Allan Barton)
Those of you interested in this subject may wish to read Dr. Allan Barton's own article on this subject from one year ago, which I discovered while sourcing out some of the images for this.