I am not a operator by profession, however I have spent my fair share of time in the seat on TLB's, mini's and my current rig (Bradco 609 backhoe attachment).
Digging technique varies based on a number of factors including: weight of machine, strength of hydraulics and soil type and compaction and bucket shape/size and condition.
The smaller machines (Mini excavator, Skid Steer backhoe attachments) do require you to control the bucket angle of attack more closely to dig efficiently.
Larger machines with their heavier weight and higher breakout forces in both the stick and bucket allow a novice user to fill the bucket because you can force it through the soil even if the cutting edge is not at its most effective angle of attack.
Smaller machines force you to dig more “with the bucket” rather than with the stick. Think of your cutting edge like a knife. With the smaller machines you need to take longer shallower cuts to fill the bucket. In hard or difficult soils this may equate to a cut that is only an inch or two deep and may take the entire crowd stroke to fill the bucket. Worn teeth and dull cutting edges will also seriously work against you. In some cases you may even have to switch to a narrower bucket to concentrate the available digging force on a smaller cutting area.
Start with your bucket teeth pointing near straight down. Put some weight on the teeth and begin curling the bucket as you crowd the stick. Curl the bucket until the stick can crowd the bucket with out stalling.
Be careful as you crowd the stick. If you want to maintain a flat bottom on these long cuts, you will have to drop the boom and uncurl the bucket as the stick swings in past its vertical position.
One problem many beginners have is as they crowd the stick, the heel of the bucket comes in contact with the cut they are making. The cutting edge will actually be angled up. This adds a lot of friction and will cause the stick to stall. If the shape of your bucket has a lot of flat in the floor directly behind the cutting edge it will have a lot more friction if it is pushing on the cut soil.
In soft soil you want the teeth to approach the soil at 30-45 degrees. Harder soil requires a flatter the angle of attack at the cutting edge. Just make sure the heel of the bucket is not dragging.
Also pay attention to the geometry of the boom and the stick. On traditional straight boom/straight stick machines, you will have maximum digging force at the point where the boom and stick are at right angles to each other. Generally crowding forces vary from low to high to low as the stick and boom go from fully extended (lower force) to 90 degrees (highest force) to stick fully retracted (lower force).
To achieve maximum cutting force in the most difficult digging conditions you may have to restrict your digging to shallow cuts, nearly flat angle of cut and a short crowd near the 90 degree stick to mast angle. Smaller bucket, proper teeth and sharp cutting edges can all contribute to efficient digging.