Basic Explanation & Setup

Horizontal stack (or Ho-stack for short) is other most prevalent offensive structure next to vertical stack. Whereas vertical stack cleanly designates a structure and order for cutters to cut from, as well as clear lanes to cut into, ho stack spreads the cutters more loosely across the field in exchange for giving the active cutters more freedom in regards to where they start and direct their cut. Below is a diagram of the standard setup and a image of it use in a real game.

In the standard Ho-stack setup, there are three handlers and four cutters in contrast to two handlers/five cutters in vertical stack. The cutters are spread out horizontally across the field giving much more space for the center cutters (#2 & #3) to cut into. When cutting, teams will often designate a pattern for pairs of cutters to move along. The most common pattern is a triangle or Diamond pattern, which is shown below.

Which cutters are active is determined by the horizontal positioning of the disc on the field. If the disc is toward the center of the field, the two middle cutters would be the active pair. If the disc is toward the sideline, the two players closest to the near sideline would be the active cutters. 

The remaining cutters main goal is creating space for the active cutters by staying out of the way and looking for opportunities to continue flow once the active cutters have either received the disc or to fill in as new active cutters if the initial cuts have been looked off. 

For a more nuanced explanation of cutting space in ho-stack, I highly recommend checking out this fantastic article from ultiworld: https://ultiworld.com/2019/10/02/understanding-strong-weak-space-horizontal-stack/