Snares don't stack with each other, the biggest one overwrites the small ones.

The effect this has is on builds with a move speed multiplier higher than 1.0, so builds that have access to Major/Minor Expedition, Steed mundus, swift jewelry, etc. As already explained, the old move speed equation was basically:

moveSpeed = baseSpeed * (1.0 + speedBuffs + sprintSpeed * sprintSpeedBuffs) * (1.0 - snare)

Where 'baseSpeed' is the base movement speed, 'speedBuffs' are the sum of all speed buffs (Major Expedition (30%, 0.3) + Minor Expedition (10%, 0.1) + 3x swift (18%, 0.18), etc), 'sprintSpeed' is the base sprint speed multiplier (40%, 0.4), 'sprintSpeedBuffs' are the sum of all sprint speed buffs, and 'snare' is the largest snare affecting you.

To make it easier to deal with, we'll drop a few parts of this, so it looks like:

moveSpeed = (1.0 + speedBuffs) * (1.0 - snare)

Lets take this equation, and figure out what the final value would be between having Major Expedition and not having Major Expedition, while under the effects of a 40% snare.

moveSpeed = (1.0 + speedBuffs) * (1.0 - snare)

// Base speed, without Major Expedition:
moveSpeed = (1.0 + 0.0) * (1.0 - 0.4)
moveSpeed = 1.0 * 0.6
moveSpeed = 0.6

// Buffed speed, with Major Expedition:
moveSpeed = (1.0 + 0.3) * (1.0 - 0.4)
moveSpeed = 1.3 * 0.6
moveSpeed = 0.78

Now, this may not seem that bad, but lets remove the one-minus part of the snare calculation, so we're just looking at the snare:

// Base speed, without Major Expedition:
moveSpeed = (1.0 + 0.0) * 0.4
moveSpeed = 1.0 * 0.4
moveSpeed = 0.4

// Buffed speed, with Major Expedition:
moveSpeed = (1.0 + 0.3) * 0.4
moveSpeed = 1.3 * 0.4
moveSpeed = 0.52

A 40% snare, actually slows you down by 52% with a 1.3x move speed multiplier, and so is 12% more effective with a 1.3x move speed multiplier. This difference will get larger as you look at higher and higher move speed multipliers.

Essentially, under the old multiplicative equation, snares disproportionately more as you stack more move speed.

Now lets look at the new additive equation, which I assume is roughly:

moveSpeed = baseSpeed * (1.0 + speedBuffs + sprintSpeed * sprintSpeedBuffs - snare)

// Or, if we simplify it down to just speed buffs and snare:
moveSpeed = (1.0 + speedBuffs - snare)

Okay, now lets plug the same numbers into this new equation:

moveSpeed = (1.0 + speedBuffs - snare)

// Base speed, without Major Expedition:
moveSpeed = (1.0 + 0.0 - 0.4)
moveSpeed = 1.0 - 0.4
moveSpeed = 0.6

// Buffed speed, with Major Expedition:
moveSpeed = (1.0 + 0.3 - 0.4)
moveSpeed = 1.3 - 0.4
moveSpeed = 0.9

As you can see, base speed is unchanged, but buffed speed is now 0.9, or 90%, instead of 0.78, or 78%. With this new equation, a 40% snare will always slow you down by 40%, regardless of your move speed multiplier.

The great thing about this is, not only does this help balance (IMO you shouldn't be punished disproportionately harder by snares the more move speed you stack, a 40% snare should always be a 40% snare), but it also makes the movement system more intuitive (it's easier to make sense of additive than multiplicative, you would think logically that a 40% snare will slow you down by 40% with 130% move speed, instead of 52%), and is is actually a little bit faster (multiplicative has 2 operations versus the 1 of additive, and the additional operation, ie a multiply, is a bit slower than an addition/subtraction).

Bit of a technical wall of text, but hope this clears it up a bit.