Provided you hold the bag by the handle directly above the coffee cup then you have a pendulum effect where the only force experienced by the liquid is in a direction perpendicular to its surface which will therefore remain flat. If you hold the cup in your hand you will be very unlikely to avoid producing a sideways force which will cause sloshing and spillage.
This product is not the same as a paper bag however, because:
- It features a non-slip base to prevent sliding
- It has a flexible finger handle to prevent sloshing
The manufacturer explains this on their web page:
SpillNot said:
What happens when a person applies a horizontal acceleration to the handle (upper end of the ‘thread’) of the SpillNot? Because the thread is very flexible, no lateral force is applied to the saucer and thus no lateral force is applied to the cup. Instead, as the top of the thread moves horizontally, the saucer and cup swing up slightly in the opposite direction and the thread exerts a force on the cup through the saucer causing acceleration in the only direction in which the thread can exert a force, toward the upper thread end that is held.
This isn't the same as a thick paper bag, where this lateral force is applied.
This is a photo of an LNER paper bag as an example. I am sure Avanti's are similar but as they're just boring brown bags it's harder to find a photo! :
(nicked from this ebay listing:
https://www.ebay.co.uk/itm/305014294559 !)
Because the handles are rigid, the lateral force SpillNot prevents is not prevented, and because it doesn't feature a non-slip base for the cup and it is twice the width of a hot drinks cup, it doesn't prevent sliding.
Also in the SpillNot promo video, they do imply that some of their maneuvers take practice. I suspect not many people practice swinging a hot drink in a paper bag round, but I suspect most people are fairly adept with carrying a cup in their hand.
And realistically if this is such a significant problem (I suspect it's not otherwise coffee shops at stations etc would also serve them in bags) there are more sensible solutions such as having a closed lid on hot drinks, just like McDonald's and other takeaway coffee places do.