Two ovals show where the axillary buds below the tip are starting to elongate. Instead of having just one cane to produce fruit, you now will have 2 branches.
For floricane-fruiting blackberries, you should be tipping them about 6" below the top wire. This will enable you to redirect the growth in a "horizontal direction" and increase your fruiting area for next year. I use "" around horizontal because the canes are growing vertically in this picture and will do so in your field. Later in the year, they will start to bend over and will form the framework for next years fruiting laterals, and be more or less horizontal with some strategic training.
For primocane-fruiting types, we have found that tipping them at about 3-4 ft., will result in higher yields. We have yield data and a published paper that shows all this data, however, validation in the field is always great. Last week I was visiting a grower that had some Prime-Ark 45. He tipped some plants (on my recommendation) and did not tip some, just to see what happened. He told me last week, yields were much better when the plants were tipped. The picture above is from his field, where all plants were recently tipped.
Also see last years post on this subject: