The idea is to get a large rigid set of bearers/bedplate directly bolted/welded to a massive and rigid part of the hull (often via wood to absorb a bit of high-frequency vibration and add some damping) so that the bearers/hull don't flex -- though as anyone who's been on a boat fitted with a Bolinder will testify another view is that the whole boat doesn't "absorb" the vibrations so much as spread them out throughout the whole hull, which can shake badly in some places at some rpm -- or indeed have the whole stern shake up and down by several inches at tickover (well, bonkover...) going by one such boat I was in a lock with... 😉
It works with slow-speed engines especially if well-balanced -- IIRC the Sabb has an internal balancer shaft to cancel out the vertical secondary vibration -- because the out-of-balance forces which cause vibration go up with the square of rpm, and the resulting vibrations are quite low-frequency so not so offensive. With higher-speed modern engines the vibrations are higher-frequency and would tend to trigger rattles and buzzes all over the boat, so flexible feet are used instead of rigid mounts. But even so there's quite a bit of vibration gets past the feet into the hull which often causes noise and vibration inside the cabin which is worse when the boat is travelling (higher rpm than idle) -- and generators do the same, even with internal flexible mounting feet.
The fix for this is the same one that is used for generators in noise-critical places like hospitals, mount the generator on a massive inertia frame which itself sits on flexible feet -- the vibration then stops at the inertia frame instead of being transmitted into the building/boat. This works best with soft feet but is more difficult with engines than generators because of the varying rpm, increased engine movement (Aquadrive or similar essential) and the need to avoid creating new and bad resonances between the engine and inertia frame at some rpm -- and also the inertia frame needs to be at least as heavy as the engine/gearbox which means several hundred kgs.
We did this with my generator, it was a PITA (and costly!) to both build and fit in underneath -- the frame weighs about 200kg (almost 2 inches of steel plate!) and is mounted on soft Aquadrive feet, and the extra weight caused some ballasting issues -- but the reduction in vibration and noise inside the boat when it's running is very obvious compared to normal generator mounting.
None of which matters to most boaters who are not installing an onboard generator, but those who are could consider doing this if they value peace and quiet... 😉