@dancer_20 the straight edges technique and the flat alpha technique are two separate techniques.
The straight edges technique is making bf knots on the left edge and fb knots on the right edge. You can do this technique both in combination with the flat alpha technique and without it. You can also do it when there is a colour switch at the edge.
The flat alpha technique is a way to do your colour switches. You can do it if there is a colour switch at the edge but in this case the old leading string will stick out at the edge. For this reason most people do the regular colour switch instead at the edge to avoid strings sticking out.
@Piglin276 to make a colour switch at the edge instead of using the flat alpha colour switch you need to use the regular colour switch. MashaKnots explains this towards the end of her video on the flat alpha technique. If you are using the straight edges technique don't forget that you need to make two bf or fb knots right after each other.