In a piston engine the combustion starts just before TDC and ends just after TDC. Between these two points there is very little movement of the psiton, so the volume of the gas is virtually constant. The addition of heat energy at constant volume produces a large increase in pressure. It is this high pressure which pushes the piston down the cylinderduring the power stroke.
In a gas turbine engine the situation is totally different. If we were to produce a big pressure rise in the combustion chamber it would stop the air from flowing out of the back of the compressor. This would cause the compressor to stall/surge.
So in a gas turbine engine we burn the mixture at constant pressure. This means that we allow the gas to expand as we heat it. This causes it to accelerate towards the turbines.
So we say that combustion in a piston engine is at constant volume, while combustion in a gas turbine engine is at constant pressure.