Short version -
Semi-stand development is also sometimes referred to as Extrem Minimal Agitation (EMA). This process uses highly dilute developer for an extended period of time. For example, when developing with pyro developers, normal dilution is usually 1:2:100 or 2:2:100 (part A: part B: water), and normal development times with 5 seconds of agitation every 30 or so coming in somewhere around 10-15 mins depending on film/exposure. With semi-stand/EMA, you would use 1:1:175 or thereabouts, and you would give approximately 5 seconds of agitation every 15 minutes or so, with development time somewhere around 1 hour.
The advantages of this are that by allowing the developer to exhaust locally on your film. The lack of agitation causes developer byproducts to build up on the film surface. This causes an increase in contrast on an extremely localized level. This is often referred to as micro-contrast. This creates an appearance of overall increased sharpness. When done properly, your film will look like etched glass when you examine the emulsion side of the negative. You will need to consider if you are going to do this before you shoot, as it does not work for all subjects. A subject with large areas of continuous tone will not benefit from this. A subject with already high inherent overall contrast may not benefit. A subject with very low inherent overall contrast and high detail will benefit greatly.
Because of the extremely extended development times, stand/semi-stand/EMA is an excellent process for controlling extremes of contrast - I know this sounds like it contradicts what I just said, but bear with me. If you have a subject, like an interior, where the overall scene contrast is fairly low, but the windows are six or eight stops brighter than the rest of the room, you can more easily adjust for reducing the negative density in the window areas because you can cut back on the total development time without impacting the shadow and mid-tone development.
The downside, in addition to the very long development times, is that you run the risk of bromide drag on the film, creating areas of uneven development, most noticeable in areas of continuous tone, like skies. It is also best done with large volumes of chemistry, and single or a very few negatives at a time. I know people who have made special tanks for developing single sheets when they are working with ULF format negatives. If you are only doing 4x5 or 8x10, deep tanks with hangers are a very viable alternative. The other downside is that unless you have a method for lightproofing your development tank, you're stuck in the darkroom for an hour with the lights out, waiting for the next fifteen minute increment to transpire so you can agitate your chemistry for five seconds.