You only have to look at the graph below showing sea level rise since 1880 to see that it has accelerated from about 1mm/year at the end of the 19th century to about 3mm/year at present.(from CSIRO).
If you take a closer look at recent sea level rise you’ll see that it has been very consistent, only deviating from the trend line by about 10mm at any time.
So if you were unscrupulous, and wanted to try to make it look like sea level rise had decelerated what could you do? You could split the series at a point where sea level was above the trend line and compare trends before and after. this is what Klaus-Eckart Puls did (green line added by me):
Of course, you could achieve the opposite effect by splitting at a point in time where sea level was below the trend line. Note that the trend for the first half, 3.5mm/year isn’t significantly different from the overall trend and that the latest measurement lies on the trend fitted to the first part of the data (the green line above).