There are combine heads that force air back into the platform to assist in reducing harvest as well as other types of heads.
However there are some simple rules to follow. I found the following excerpts from a Missouri article useful during harvest to capture the losses that may occur during harvest.(Missouri Department of Agricultural Engineering)
Your best guide for correct combine adjustment is your operator's manual.
Remember that more than 80 percent of the machine loss usually occurs at the gathering unit. The following suggestions will help keep these losses to a minimum.
• Make sure that knife sections, guards, wear plates and hold-down clips are in good condition and properly adjusted.
• Use a ground speed of 2.8 to 3.0 miles per hour. To determine ground speed, count the number of 3-foot steps taken in 20 seconds while walking beside the combine. Divide this number by 10 to get the ground speed in miles per hour.
• Use a reel speed about 25 percent faster than ground speed. For 42-inch-diameter reels, use a reel speed of 11 revolutions per minute for each 1-mile-per-hour ground speed.
• Reel axle should be 6 to 12 inches ahead of the cutter bar. Reel bats should leave beans just as they are cut. Reel depth should be just enough to control the beans.
• A six-bat reel will give more uniform feeding than a four-bat reel.
• Complete the harvest as quickly as possible after beans reach 15 percent moisture content.
• A pick-up type reel with pick-up guards on the cutter bar is recommended when beans are lodged and tangled.