About 18 months ago FINA approved starting blocks with an angled wedge-shaped footrest on the back edge. The idea is that athletes will have the advantage of producing more force from the blocks and therefore faster starts/races. In addition to the wedge some manufacturers are able to 
provide side handles on the blocks that athletes can use to generate speed. Such handles were part of the custom blocks at IUPUI that were installed 30 years ago and have been allowed for many years. The measure of the amount of advantage of the adjustable footrest is not conclusive but testing indicates that the handles offer significant advantage. USA Swimming and recently the NCAA voted to allow the wedge style blocks but NFSHSA has not yet approved them. KDI Paragon, SR Smith and Spectrum, have wedge-style blocks with side handles. Omega has wedge-style blocks. Anti and Kiefer only offer side handles only. Older blocks by KDI Paragon and Spectrum can be retrofitted to have a wedge and handles.
There are negative aspects to the wedge. Different athletes will want different locations so wedges must be easily movable. Some athletes do not like the wedge and choose not to use it. Some athletes may not be used to the manufacturer’s method of adjustment. Delays in meets may be common. The wedge also may need to be safely removable during relay starts. Then, where does it go? Who replaces it? With rear step starting blocks stepping over the wedge can be a safety concern. With side step starting blocks stepping over the handles may be a problem.