After the rounds have dropped into the chambers, release them by whatever method your speedloader brand requires. If you’re using a Safariland or SL Variant, simply push the body of the speedloader toward the cylinder and the rounds will release.
If your speedloader is an HKS or similar design, insert the rounds into the cylinder, then grab the knob and twist it to release the rounds.
As the rounds drop from the speedloader, pull it straight back out to make sure all of the rounds have cleared, then drop it over the side of the gun. This eliminates another failure point of the reload procedure: rounds binding in the loader.
It’s not uncommon to have the speedloader tilt just a bit and cause a round or two to bind inside the loader. When this happens the round(s) don’t clear the loader, and if you attempt to close the cylinder you’ll trap that round(s) and the speedloader against the frame of the gun. You’ll then need to open the cylinder, grab the speedloader, shake the round(s) loose and finally finish the reload. Yes, it’s time consuming and aggravating, which is why I strive to avoid it by lifting the loader and allowing the rounds to drop free, then tossing the speedloader away.
The two most common and widely available speedloaders on the market are the Safariland, which use a push-to-release mechanism, and the HKS, which releases by turning a knob. (There are other examples of each style, such as the push-type SL Variant and the turn-type Five Star, but Safariland and HKS are by far the most readily available. They’re the least expensive in their respective categories as well.)
In general I much prefer the Safariland, simply because they’re more intuitive in use. You’re already grasping the speedloader by its body and pushing the rounds into the cylinder; releasing the rounds from the Safariland is a simple continuation of that movement. The HKS style, on the other hand, requires you to release your grasp of the body, find the knob and grab it, then twist it in the proper direction to allow the rounds to drop into the cylinder. I don’t consider that as intuitive as simply continuing pushing the way you’re already pushing.
I’ve also found that the Safariland speedloaders, when properly loaded, are more secure. They’re less likely to accidentally release in a pocket or when dropped.