Depends on specific lock design. Some intentionally create a strong forward pull on the boat, so long as it us close enough to the top. The closeness depends on the lock! In some cases, the pull us too strong for the boat in full reverse to resist and the result is a substantial impact in cill or gate. It us quite easy to muss tge pull staring until it us too late to do anything about it.
My understanding us that this was intentional for full length boats, not many short boats anyway.
In some locks there is a specific bumper board that hangs in front of the cill. I am not sure whether it is to limit the effect of unintentional impacts or whether it was accepted practice.
Although the practice is called riding the cill, I suspect that it is a misnomer in the sense that the cill is the angled shape that the gates fit against, below water, to create a seal, along with the way in which the champhered edges come together vertically. In most cases, the riding is against the main bulk of the lock below the cill. This arrangement can be seen clearly in deeper locks where the lower water level is below the cill.
I also suspect that damage is largely done on an impact rather than sustained pressure on what is a large wall. So, if tte steere comes into the lock and reaches the far end at 'zero' speed then not much happens.
In many narrow locks the gate itself is protected with a thick steel plate, again seemingly intended to mitigate the effect of a boat's bow on it. It also ensures that the bist does not catch under the balance beam.
There are various reasons why the practice is helpful to traditional boaters which are different to today's boating scenario.
How many boats will enter a full lock from above and fail to come to a full halt before reaching the bottom gates. Impact here is more likely to have a serious consequence.
Hanging too far back risks either the rudder or something else being trapped between the gates, leading to a sinking.
Overall, there many ways of working a lock, whether single handed or not. I do not believe that any if them are without some downside so it is up to the steerer to make their own risk assessment.