Trying to find a buried person by pushing a stick into the snow is caveman-style searching. However probing is reliable. Yes, you may mistakenly think you hit the victim when you haven't (a false positive), but your probe will never pass through the victim (a false negative).
There are three variations of probing: spot probing, an organized probe line, and transceiver probing. Spot probing involves probing high-probability areas. This includes downhill of clues (e.g., skis or a snowmobile), uphill of catchments (e.g., trees, bushes, rocks), near the toe of the avalanche, etc. Spot probing is most commonly used when you don't receive a signal from an avalanche transceiver (either because the victim wasn't wearing a transceiver or because it is no longer transmitting).
An organized probe line is used when the victim cannot be located using a transceiver or by other means (e.g., spot probing or dogs). A probe line requires significant manpower and is a slow process.
Transceiver probing takes place after completing a transceiver fine search. It is an important step to confirm that you are in the correct location and that the transceiver is attached to a body.
A probe that hits a human body, or a backpack, has a slightly springy, "boungy" feel. Don't press too hard. Your job is to locate rather than perforate. Branches can also have a springy feel (and occasionally I've smelled the scent of pine on the tip of the probe). A probe that hits skis, a boot, or a snowmobile has a much different feel. As you would expect, rocks have a hard feel and can make a clicking sound. Ice layers (and sometimes frozen dirt) can have a "sticky" feel where the tip of the probe is momentarily held by the ice. If you are unsure whether your probe is contacting a body, probe several times around your initial probe hole to gauge the size and "feel" of the object.
When you find the victim with your probe, leave it in place and begin shoveling. It's important that leave your probe in place so you shovel in the correct location. The probe is also a good reminder to not stand on top of the victim.
When purchasing an avalanche probe, consider only a dedicated sectional probe. Convertible ski pole probes are okay as a backup, but they are too short, too difficult to assemble, and they are lousy at penetrating dense avalanche debris.
Your dedicated probe should have an assembled length of at least 220 cm (although ensure the stowed probe fits in your favorite pack). Shorter probes will require that you bend over each time you insert the probe. Long probes are necessary only when probing an unsurvivable deep burial and are more appropriate for patrollers and search and rescue teams. Etched depth indicators and contrasting segments are helpful for determining burial depth and measuring snow pits.
Look for a simple assembly mechanism that locks, by pulling on a handle or cable, and that does not require any additional locking. Avoid probes that require you to position a knot in a tiny groove or that requires you to manually spin a knurled nut, and don't buy any probe where the tubes can accidentally come off the cable/string. Make sure you can reliably deploy the probe with your typical winter handwear.
The Pieps iProbe is a unique probe that can temporarily silence a Pieps transmitter.
Once you complete the fine search and start probing, trust your probe and avoid the temptation to return to your transceiver. The time you "waste" probing a larger area is usually small compared to the time you will waste returning to a transceiver search.
Hold the probe firmly with both hands (don't cling to your transceiver with one hand—you are probing now). When probing for a transceiver signal, insert the probe at 90° relative to the snow, not relative to gravity. This is because your transceiver will take you to the closest location relative to the transmitting beacon, which is not necessarily directly above the victim. If you mistakenly hold your probe vertically the transmitter will usually be uphill of your first probe. (When spot probing, the probe can be held at any angle; when used in a probe line, the probe should be held vertically.)
You should first insert your probe at the location where your avalanche transceiver reported the strongest signal (i.e., the shortest distance). Subsequent probes should be in circles (or a spiral as proposed by Manuel Genswein in 2002) from this center point at no more than 25 cm (10 inch) intervals. This small distance between holes ensures that you will not miss the victim. It is better to "waste" a little time probing tightly than to miss and have to re-probe the entire area.
If your fine search was decent, you'll usually find the victim within 5 to 10 insertions. Even if you encountered a spike (which is very uncommon with modern, three-antenna transceivers) or your fine search was sloppy, continuing to probe every 25 cm (10 inches) will eventually locate your victim. For most people, continued probing is much faster than trying to interpret the spikes. (My rough time estimate is that probing a one-meter radius at 10-inch intervals will take less than five minutes.)
When you find the victim with your probe, leave it in place and begin shoveling. Leaving the probe in place will help you stay on-target as you shovel and will remind you to not stand on the victim which might compromise his airway.
A "probe line" involves a row of rescuers repeatedly inserting avalanche probes and advancing. It is an effective albeit slow way to search an avalanche. A probe line usually has a leader, probers, and possibly two people who manage a guidon cord. The probers can be bystanders who are trained on the spot.
The following slideshow illustrates the probe line process. These instructions assume 50cm (20 inches) between probe holes. This tight spacing is a good when time isn't as crucial (i.e., after a several hours of searching) and you want a relatively high level of accuracy.
Probers. It is much easier on the probers' backs if they face uphill. It also gives them better situational awareness—not that they could outrun an avalanche. The probers can achieve five-foot spacing by standing palm-to-palm.
Lines People. The lines people can hold a guidon (pronounced "guide-on") cord that is typically marked (either with paint, knots, or ribbon) every 50 cm (20 inches). The markings help keep the probers in a line and ensure proper spacing, but the spacing will be good even without a cord provided the probers maintain the five-foot spacing.
The most important task performed by the lines people (and this can be done by the probers on the ends of the probe line) is to periodically place flags adjacent to the outside probe hole. Knowing the terrain that has been probed is critical as the incident commander adapts the search plan.
Leader. The leader is ultimately responsible for the entire probe line search. In addition to giving the verbal probing commands, the leader must ensure that proper spacing is maintained between probers and that the probers remain focused. The leader can observe the heights of the inserted probes to gauge the underlying terrain versus a possible probe spike. Additionally, the leader may need to train bystanders so they can participate in the probe line.