Paddling (of any kind) relies on Newton's third law of motion, which states that for every action there is an equal and opposite reaction. This means that the force you apply on your paddle results in an opposite directional force for the boat.
|Picture courtesy of HowStuffWorks|
Getting a little more advanced...
|Time lapse of 3 phases of the stroke|
|Animation of stroke. Courtesy of Wikimedia Commons.|
As stated just above, slippage is the phenomenon where the force applied on our paddle is no longer completely moving us forward, but also moving the water backwards. This can be visibly seen as waves coming off of the paddle. I know that some paddlers use this as a cue that they're putting in a lot of force (which they definitely are), but unfortunately they are sacrificing on efficiency.
There are times when slippage is hard (if not impossible) to avoid; case in point, the first strokes of the start. Since the boat is at a stand-still, it takes a lot of force to get it up and moving. This amount of force can normally be in excess of what the water tension will allow and your paddle will slip, so there's no way to apply the appropriate amount of force without sacrificing a little bit of it towards slippage. But since we're talking about only a few strokes in an entire race, I'm completely fine with that. What I'm not a fan of, however, is slippage during the race (especially longer races, where we can't spare wasted energy).
How to prevent it
#1) Bury your paddle
You can avoid slipping by making sure that your paddle is in contact with the largest surface of water possible when applying pressure. This means you have to bury the whole paddle blade, especially if you're applying a lot of pressure. If you only have half your paddle in the water, then you're limited at half the amount of force possible before slippage occurs.
#2) Apply force gradually and throughout the whole stroke
Shooting all your force into the water in a single shot will increase your chances of slippage. Instead think about balancing the pressure during the entire stroke; beginning with the least amount of pressure at the catch and maximum amount at the power phase (when the paddle is perpendicular with the water). This also ensures maximum forward directional force. N.B. I'm not encouraging "soft catches" here, I'm saying that the pressure at the power phase should be greater than that at your catch.
#3) Clean up your entry
A good stroke starts with a good entry, and making sure you have a clean entry can be the first step at minimizing slippage. A missed catch can be identified by its signature "ker-plunk" sound. The reason you hear that is because the paddle is actually trapping an air pocket underwater. This air pocket contributes to slippage because the paddle moves through air much easier than water. Eliminate the "ker-plunk" to eliminate the slippage, and your teammates will probably thank you for less splashing too.
#4) Eliminate the bicep flex at the exit
In most of the scenarios that I've witnessed slippage, it occurs most heavily at the end of the stroke. Paddlers will have the urge to finish their stroke with a bicep pull just before the exit, which allows them to apply a quick burst of force into the water. While it might seem like a great idea, this sudden burst usually results in slippage, and worst, the slippage is usually in the form of scooping water backwards and onto the lap of the paddler behind you. Furthermore, the angle of the blade when executing a bicep pull is driving the boat downwards (!). Consider instead of ending your stroke with tricep extension to "press" off the exit, rather than to "pull" into a negative exit.
#5) Don't hold back pressure, just be smarter about where to use it
What I definitely don't want to happen here is that paddlers become overly self-conscious about slippage and will hold back on their stroke. I want you to just be smarter by acknowledging when it happens. If you're paddling at 80% pressure during the piece, and on power series (100% pressure) you are slipping a lot (and can't correct it with #1-4), then consider making it a more incremental increase and maintain more pressure during the piece. E.g. Increase to 85% pressure during the piece, and decrease to 92-95% on the power series. The overall energy output may be the same, but the total forward directional force will increase.